OT: DVD recording/playback...

[JR North]

\rant
OK, I haven't thought this through completely before posting, but...
I have 2 DVD recorders, one Toshiba DVD-R, one Sanyo DVD+R/DVD+RW. Both
work well.
Movies recorded on either won't play properly on DVD 'players' like my
Toshiba 9"TV/DVD or my new Audiovox 15" LCD TV/DVD. Recorded Discs from
both recorders will start OK, but 5-15 minutes into the first title,
start displaying digital artifacts, then eventually stall. Neither will
play a second title.
I know various manufacturers state that their units may not play discs
recorded on other units. What's up with that? Just what is 'Versatile'
about DVD that makes it so un-versitile?
Before I start really raving on the absurdity of this bullshit, and just
exactly what's wrong with the consuming public that this is tolerated,
let me digress.
/rant
Are there variables like disc brand, Xspeed rating, recorder
settings(SP/LP, etc),that have been known to influence this fault in a
positive direction? I don't really want to embark on a lengthy
experiment by buying all the different disc choices and testing, only to
come up unsuccessful.
I bought both DVD players to play my recorded DVDs on my boat. As of
now, this plan is defunct.
More information later...
Any input?
JR



--
--------------------------------------------------------------
Home Page: http://www.seanet.com/~jasonrnorth

 

[Jerry]

On Sep 26, 9:31 am, JR North <junkjasonrno...@bigfoot.com> wrote:

Are there variables like disc brand, Xspeed rating, recorder
settings(SP/LP, etc),that have been known to influence this fault in a
positive direction? I don't really want to embark on a lengthy
experiment by buying all the different disc choices and testing, only to
come up unsuccessful.

Disc brand is a definite variable. When I first started recording
DVDs, I bought a big spindle of GQ (Great Quality - HAH!) blanks from
Frys Electronics. Things that I recorded on them would be OK at the
start, then start breaking up at the end. DVDs record from the inside
to the outside, so the linear speed past the laser is faster at the
end, thus more difficult to write and read reliably. Those GQ discs
just wouldn't work reliably. Switched to using Memorex DVD-Rs, been
working OK for me for a few years now, tossed the rest of the GQs.
Tried +Rs once, but one of the players in the house won't recognize a
+R, so sticking with Memorex -Rs.

A little bit of googling might lead you to a website that rates the
various brands of DVDs.

Hope this helps,
Jerry

 

[Jerry]

On Sep 26, 5:36 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:

That isn't true, I think. CD and DVD use constant linear velocity (CLV)
servos, rather than constant angular velocity (CAV). This means that as the
laser tracks from the centre to the outside of the disc, the rotational
speed of the disc is slowed, to maintain a constant writing or reading speed
of the track, past the optical block.

If you can believe everything you read on the internet, I saw the
following on the DVD Demystified website : http://www.dvddemystified.com/dvdfaq.html#1.1.
Leads me to believe that there are both CLV and CAV drives out there.

--

In order to maintain constant linear density, typical CD-ROM and DVD-
ROM drives spin the disc more slowly when reading or writing near the
outside where there is more physical surface in each track. (This is
called CLV, constant linear velocity.) Some faster drives keep the
rotational speed constant and use a buffer to deal with the
differences in data readout or writeout speed. (This is called CAV,
constant angular velocity.) In CAV drives, the data is read or written
fastest at the outside of the disc, which is why specifications often
list "max speed."

Jerry

 

[IanM]

Arfa Daily wrote:

"Jerry" <jerry_maple@hotmail.com> wrote in message
news:24a7ffbc-569b-45fa-9030-a656f406d641@q9g2000hsb.googlegroups.com...
On Sep 26, 5:36 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:


That isn't true, I think. CD and DVD use constant linear velocity (CLV)
servos, rather than constant angular velocity (CAV). This means that as
the
laser tracks from the centre to the outside of the disc, the rotational
speed of the disc is slowed, to maintain a constant writing or reading
speed
of the track, past the optical block.



If you can believe everything you read on the internet, I saw the
following on the DVD Demystified website :
http://www.dvddemystified.com/dvdfaq.html#1.1.
Leads me to believe that there are both CLV and CAV drives out there.

--

In order to maintain constant linear density, typical CD-ROM and DVD-
ROM drives spin the disc more slowly when reading or writing near the
outside where there is more physical surface in each track. (This is
called CLV, constant linear velocity.) Some faster drives keep the
rotational speed constant and use a buffer to deal with the
differences in data readout or writeout speed. (This is called CAV,
constant angular velocity.) In CAV drives, the data is read or written
fastest at the outside of the disc, which is why specifications often
list "max speed."

Jerry

That's interesting Jerry. I've never before seen any writeups on CD/DVD
rotation control systems, which suggest that anything other than CLV servos
are used. I suppose that if you keep the angular velocity the same, and vary
the data transfer rate, then in terms of what's actually recorded, that
amounts to the same thing, achieved by an alternate method. That being the
case, then your original point about the writing speed at various places on
the disc makes a lot more sense. Thanks for the info. I wonder what exactly
is the thinking behind doing it in what seems to be a more complex way that
requires a chunk of buffer memory, and why this should be (apparently)
associated particularly with 'high speed' writers ? Time for some more
research, I think !

Arfa


A CLV drive will need a significantly better motor and some care in the

mechanical design to prevent resonances. A CAV drive can have much
cheaper mechanicals and can handle random access. (A CLV drive has to
diddle the motor speed on every seek till the tracking servo locks again
- slow and clunky.)

Add in marketing factors like selling on buffer size and that a CAV
drive claiming a particular access speed may only run that fast on
AVERAGE across a maximum length disk, and I'd be surprised to find any
new ROM drives that still do CLV. Recorders are a different matter, Its
gotta be easyier to get a consistant result at CLV and I wouldn't be
surprised to see both strategies in the same high end drive depending on
recording speed selected and type of disk.

 

[Franc Zabkar]

On Tue, 30 Sep 2008 09:02:08 +0100, IanM <Invalid@totally.invalid> put
finger to keyboard and composed:

A CLV drive will need a significantly better motor and some care in the
mechanical design to prevent resonances. A CAV drive can have much
cheaper mechanicals and can handle random access. (A CLV drive has to
diddle the motor speed on every seek till the tracking servo locks again
- slow and clunky.)

Add in marketing factors like selling on buffer size and that a CAV
drive claiming a particular access speed may only run that fast on
AVERAGE across a maximum length disk, and I'd be surprised to find any
new ROM drives that still do CLV. Recorders are a different matter, Its
gotta be easyier to get a consistant result at CLV and I wouldn't be
surprised to see both strategies in the same high end drive depending on
recording speed selected and type of disk.

ISTR a discussion where someone claimed that his recorded discs were
of non-uniform colour, which suggested that the laser's power was not
being appropriately varied as it moved from hub to circumference, and
that the recording mode was CAV, not CLV, at least in one section of
the disc.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Jeroni Paul]

On 30 Set, 10:02, IanM <Inva...@totally.invalid> wrote:

A CLV drive will need a significantly better motor and some care in the
mechanical design to prevent resonances.  A CAV drive can have much
cheaper mechanicals and can handle random access. (A CLV drive has to
diddle the motor speed on every seek till the tracking servo locks again
  - slow and clunky.)

Add in marketing factors like selling on buffer size and that a CAV
drive claiming a particular access speed may only run that fast on
AVERAGE across a maximum length disk, and I'd be surprised to find any
new ROM drives that still do CLV.  Recorders are a different matter, Its
gotta be easyier to get a consistant result at CLV and I wouldn't be
surprised to see both strategies in the same high end drive depending on
recording speed selected and type of disk.- Amaga el text entre cometes -

Well, CD-ROMs for PCs appear to use CAV since the disk always rotates
at the same speed, except when it is having difficulties to read.
But some cheap set-top DVD players use CLV. In one I repaired the
motor was a standard DC brush motor and a double power op-amp driver
chip. The controller set the speed by adjusting a voltage, and it
could not only accelerate but also brake by taking away power
generated in the motor. Without the braking it did play but would
stall at the first seek to the end of the disk.

 

[Arfa Daily]

"Jeroni Paul" <JERONI.PAUL@terra.es> wrote in message
news:5e65f477-e8f6-42dd-9b96-ef2beee1e956@26g2000hsk.googlegroups.com...
On 30 Set, 10:02, IanM <Inva...@totally.invalid> wrote:

A CLV drive will need a significantly better motor and some care in the
mechanical design to prevent resonances. A CAV drive can have much
cheaper mechanicals and can handle random access. (A CLV drive has to
diddle the motor speed on every seek till the tracking servo locks again
- slow and clunky.)

Add in marketing factors like selling on buffer size and that a CAV
drive claiming a particular access speed may only run that fast on
AVERAGE across a maximum length disk, and I'd be surprised to find any
new ROM drives that still do CLV. Recorders are a different matter, Its
gotta be easyier to get a consistant result at CLV and I wouldn't be
surprised to see both strategies in the same high end drive depending on
recording speed selected and type of disk.- Amaga el text entre cometes -

Well, CD-ROMs for PCs appear to use CAV since the disk always rotates
at the same speed, except when it is having difficulties to read.
But some cheap set-top DVD players use CLV. In one I repaired the
motor was a standard DC brush motor and a double power op-amp driver
chip. The controller set the speed by adjusting a voltage, and it
could not only accelerate but also brake by taking away power
generated in the motor. Without the braking it did play but would
stall at the first seek to the end of the disk.


I think you'll find that pretty much all domestic entertainment DVD players
/ home cinema units, irrespective of their cost or pedigree use CLV servos.

Arfa

 

[William Sommerwerck]

If digital disk players -- for whatever format -- were CAV, they'd have
roughly only half the recording time they do. They don't need CAV, as the
constant clock rate allows timing variations to be corrected.

 

[Arfa Daily]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:evGdndT7Rvr8jXXVnZ2dnUVZ_hOdnZ2d@comcast.com...

If digital disk players -- for whatever format -- were CAV, they'd have
roughly only half the recording time they do. They don't need CAV, as the
constant clock rate allows timing variations to be corrected.

As I understood the description given by another poster, William, Drives
which use CAV servos for reasons that sound pretty valid (typically those
fitted to computers), feed the data to the disk for recording, at a variable
rather than constant rate, using a chunk of buffer memory to accomplish
this. The net result is that the data is still recorded at a constant
'pitch' along the tracks, the same as it would have been if a constant data
rate, and variable spin speed had been used. Thus, when the drive has
finished recording the disk, its format will be exactly the same as any
other CD / DVD. If you think about it, this would have to be the case,
otherwise adherence to the standards laid down for CD / DVD, and
compatibility with other players, would be compromised.

Presumably, on drives which used CAV servos, the reverse principle is used
for playing back ? If so, it must all work out the same, otherwise, a
commercial disk would not play in both your CAV computer drive, and the CLV
drive in your home cinema unit.

Arfa

 

[William Sommerwerck]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Qt0Gk.19368$tq7.8191@newsfe28.ams2...

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:evGdndT7Rvr8jXXVnZ2dnUVZ_hOdnZ2d@comcast.com...

If digital disk players -- for whatever format -- were CAV, they'd have
roughly only half the recording time they do. They don't need CAV,
as the constant clock rate allows timing variations to be corrected.

As I understood the description given by another poster, William, Drives
which use CAV servos for reasons that sound pretty valid (typically those
fitted to computers), feed the data to the disk for recording, at a
variable
rather than constant rate, using a chunk of buffer memory to accomplish
this. The net result is that the data is still recorded at a constant
'pitch' along the tracks, the same as it would have been if a constant
data
rate, and variable spin speed had been used. Thus, when the drive has
finished recording the disk, its format will be exactly the same as any
other CD / DVD. If you think about it, this would have to be the case,
otherwise adherence to the standards laid down for CD / DVD, and
compatibility with other players, would be compromised.

Thanks for the clarification. I wondered why you said "CAV servos", rather
than CAV.

As far as I know, CD and DVD writers record chunks of data at a constant
spin rate. What you're suggesting is doable, but would get really messy
having to continually change the data-writing rate. And it probably isn't
necessary. See below.

Presumably, on drives which used CAV servos, the reverse principle
is used for playing back?

Not necessarily. If each chunk of data represents only a few revolutions,
the playback circuitry should have no trouble handling the slight change in
data rate.

To put it another way... It isn't necessary for a digital optical disk to be
written to at an absolutely constant data rate. The clocking and buffering
circuits should be able to handle a change of a few percent.

To give a related example... I have a Sony 601 digital processor, the only
one that can convert recordings made at 44.056kHz to S/PDIF format. An
outboard DAC has no trouble with it, despite the fact it's the "wrong"
frequency. Yes, it's only 0.1% slow, but the principle is the same.

 

[Geoffrey S. Mendelson]

Arfa Daily wrote:

Presumably, on drives which used CAV servos, the reverse principle is used
for playing back ? If so, it must all work out the same, otherwise, a
commercial disk would not play in both your CAV computer drive, and the CLV
drive in your home cinema unit.

It could just be that the microprocessor in the drive detects the current
bit rate and reads it accordingly. That way a disk can be written in
in almost any method at almost any speed and still be read.

This compensates for buggy writers, component aging, etc.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com N3OWJ/4X1GM

 

[Samuel M. Goldwasser]

gsm@mendelson.com (Geoffrey S. Mendelson) writes:

Arfa Daily wrote:
Presumably, on drives which used CAV servos, the reverse principle is used
for playing back ? If so, it must all work out the same, otherwise, a
commercial disk would not play in both your CAV computer drive, and the CLV
drive in your home cinema unit.

It could just be that the microprocessor in the drive detects the current
bit rate and reads it accordingly. That way a disk can be written in
in almost any method at almost any speed and still be read.

This compensates for buggy writers, component aging, etc.

CD/DVD/Blu-Ray are all CLV to achieve maximum storage capacity. The drive or
player can do whatever the heck it pleases in reading them.

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Arfa Daily]

"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:xISdnc2sEvHJOnXVnZ2dnUVZ_g2dnZ2d@comcast.com...

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:Qt0Gk.19368$tq7.8191@newsfe28.ams2...
"William Sommerwerck" <grizzledgeezer@comcast.net> wrote in message
news:evGdndT7Rvr8jXXVnZ2dnUVZ_hOdnZ2d@comcast.com...

If digital disk players -- for whatever format -- were CAV, they'd have
roughly only half the recording time they do. They don't need CAV,
as the constant clock rate allows timing variations to be corrected.

As I understood the description given by another poster, William, Drives
which use CAV servos for reasons that sound pretty valid (typically those
fitted to computers), feed the data to the disk for recording, at a
variable
rather than constant rate, using a chunk of buffer memory to accomplish
this. The net result is that the data is still recorded at a constant
'pitch' along the tracks, the same as it would have been if a constant
data
rate, and variable spin speed had been used. Thus, when the drive has
finished recording the disk, its format will be exactly the same as any
other CD / DVD. If you think about it, this would have to be the case,
otherwise adherence to the standards laid down for CD / DVD, and
compatibility with other players, would be compromised.

Thanks for the clarification. I wondered why you said "CAV servos", rather
than CAV.

As far as I know, CD and DVD writers record chunks of data at a constant
spin rate. What you're suggesting is doable, but would get really messy
having to continually change the data-writing rate. And it probably isn't
necessary. See below.


Presumably, on drives which used CAV servos, the reverse principle
is used for playing back?

Not necessarily. If each chunk of data represents only a few revolutions,
the playback circuitry should have no trouble handling the slight change
in
data rate.

To put it another way... It isn't necessary for a digital optical disk to
be
written to at an absolutely constant data rate. The clocking and buffering
circuits should be able to handle a change of a few percent.

To give a related example... I have a Sony 601 digital processor, the only
one that can convert recordings made at 44.056kHz to S/PDIF format. An
outboard DAC has no trouble with it, despite the fact it's the "wrong"
frequency. Yes, it's only 0.1% slow, but the principle is the same.

Accepted, but the data rate or spin rate, would be significantly different
between the start and end of the disk. If the (CAV) drive runs the disk at a
constant speed, there will be a large difference in the data rate between
the start and end, just as with a CLV drive, there is a large difference in
rotational speed between the start and end. As far as accuracy of the
writing speed goes, I would suggest that this is pretty tightly controlled,
and is probably the reason that with high speed writers, the disk is run at
a constant speed, and the data rate varied to match the writing position on
the disk between outside and inside edges of the data area. It would be a
lot easier to vary the data readout rate from the record buffer, than to try
to maintain an accurate varying (high) rotation speed, particularly on
drives that use a DC brushed motor, rather than the brushless DD types used
by better drives.

As far as CAV drives reading data back, I was being general in assuming that
the reverse principle was applied in that again, the disc is rotated at a
constant speed, and the data is processed at a constant rate, by varying the
readout speed of a chunk of buffer memory, rather than the drive switching
over to CLV for reading.

As to your point about record rates, if the disc is spun at a constant rate,
then the data must be presented at a variable writing rate, in order to
maintain a constant data pitch along the recorded track. Messy or not, it is
absolutely essential that this is done to preserve that constant pitch,
which is part of the CD / DVD specification. The difference in the lengths
of track on each rotation, between the start and end of the data area, is
simply too great for the system to be able to cope with merely as a
percentage error around the nominal data rate. This is why there is a
significant difference between the spin rate at the start and end of a disc
in a standard CLV home entertainment player, and a sophisticated servo to
take care maintaining a correct spin rate.

Arfa

 

[Michael A. Terrell]

Arfa Daily wrote:

Accepted, but the data rate or spin rate, would be significantly different
between the start and end of the disk. If the (CAV) drive runs the disk at a
constant speed, there will be a large difference in the data rate between
the start and end, just as with a CLV drive, there is a large difference in
rotational speed between the start and end. As far as accuracy of the
writing speed goes, I would suggest that this is pretty tightly controlled,
and is probably the reason that with high speed writers, the disk is run at
a constant speed, and the data rate varied to match the writing position on
the disk between outside and inside edges of the data area. It would be a
lot easier to vary the data readout rate from the record buffer, than to try
to maintain an accurate varying (high) rotation speed, particularly on
drives that use a DC brushed motor, rather than the brushless DD types used
by better drives.

As far as CAV drives reading data back, I was being general in assuming that
the reverse principle was applied in that again, the disc is rotated at a
constant speed, and the data is processed at a constant rate, by varying the
readout speed of a chunk of buffer memory, rather than the drive switching
over to CLV for reading.

As to your point about record rates, if the disc is spun at a constant rate,
then the data must be presented at a variable writing rate, in order to
maintain a constant data pitch along the recorded track. Messy or not, it is
absolutely essential that this is done to preserve that constant pitch,
which is part of the CD / DVD specification. The difference in the lengths
of track on each rotation, between the start and end of the data area, is
simply too great for the system to be able to cope with merely as a
percentage error around the nominal data rate. This is why there is a
significant difference between the spin rate at the start and end of a disc
in a standard CLV home entertainment player, and a sophisticated servo to
take care maintaining a correct spin rate.

Arfa

Arfa, variable sectors per track has been around for a long time. As
your tracks move outward, you change the clock rate of the decoder
circuit, which changes the number of sectors. Commodore was doing it
with floppies over 25 years ago.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[Arfa Daily]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:XuadnYpRG-eqo3fVnZ2dnUVZ_qrinZ2d@earthlink.com...

Arfa Daily wrote:

Accepted, but the data rate or spin rate, would be significantly
different
between the start and end of the disk. If the (CAV) drive runs the disk
at a
constant speed, there will be a large difference in the data rate between
the start and end, just as with a CLV drive, there is a large difference
in
rotational speed between the start and end. As far as accuracy of the
writing speed goes, I would suggest that this is pretty tightly
controlled,
and is probably the reason that with high speed writers, the disk is run
at
a constant speed, and the data rate varied to match the writing position
on
the disk between outside and inside edges of the data area. It would be a
lot easier to vary the data readout rate from the record buffer, than to
try
to maintain an accurate varying (high) rotation speed, particularly on
drives that use a DC brushed motor, rather than the brushless DD types
used
by better drives.

As far as CAV drives reading data back, I was being general in assuming
that
the reverse principle was applied in that again, the disc is rotated at a
constant speed, and the data is processed at a constant rate, by varying
the
readout speed of a chunk of buffer memory, rather than the drive
switching
over to CLV for reading.

As to your point about record rates, if the disc is spun at a constant
rate,
then the data must be presented at a variable writing rate, in order to
maintain a constant data pitch along the recorded track. Messy or not, it
is
absolutely essential that this is done to preserve that constant pitch,
which is part of the CD / DVD specification. The difference in the
lengths
of track on each rotation, between the start and end of the data area, is
simply too great for the system to be able to cope with merely as a
percentage error around the nominal data rate. This is why there is a
significant difference between the spin rate at the start and end of a
disc
in a standard CLV home entertainment player, and a sophisticated servo to
take care maintaining a correct spin rate.

Arfa


Arfa, variable sectors per track has been around for a long time. As
your tracks move outward, you change the clock rate of the decoder
circuit, which changes the number of sectors. Commodore was doing it
with floppies over 25 years ago.

Hmmm. I'm not sure how that stacks up with CD / DVD though, as with those
media, there are no sectors as such, just constant length data frames,
recorded to disc at a constant pitch. This being the case, I don't think
that you would be able to decode by simply varying the clock rate of the
decoder. I think, as was suggested by someone else, that you would have to
run the data through a buffer memory, and vary the clocking of the MMU to
arrive at a constant data rate input for the decoder ??

Arfa

 

[Geoffrey S. Mendelson]

Arfa Daily wrote:

Hmmm. I'm not sure how that stacks up with CD / DVD though, as with those
media, there are no sectors as such, just constant length data frames,
recorded to disc at a constant pitch. This being the case, I don't think
that you would be able to decode by simply varying the clock rate of the
decoder. I think, as was suggested by someone else, that you would have to
run the data through a buffer memory, and vary the clocking of the MMU to
arrive at a constant data rate input for the decoder ??

They are "soft sectored", which has been the same for just about every disk
drive made in the last 30 years. There were hard sectored hard drives,
which disapeared in the 1970's and hard sectored floppies, but only one
or two computers used them in smaller than 8 inch and they did not last
long once soft sectored disks became cheap and common.

They work with a gap between sectors and a header and trailer.

It would be very easy to just "listen" for a header and adjust the clock
speed until it decodes and then read until the correct sector header
starts to come in before doing anything with the data. There is no need
to buffer it and play around with the buffer.

You just have to look for a section of silence (gap) and then a particular
bit pattern (sector header) before you start decoding.

The only cost is time, it may take a revolution or two to "sync up" or not
depending upon how close the clock is to the data.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com N3OWJ/4X1GM

 

[Arfa Daily]

"Geoffrey S. Mendelson" <gsm@mendelson.com> wrote in message
news:slrngeltie.vp3.gsm@cable.mendelson.com...

Arfa Daily wrote:
Hmmm. I'm not sure how that stacks up with CD / DVD though, as with those
media, there are no sectors as such, just constant length data frames,
recorded to disc at a constant pitch. This being the case, I don't think
that you would be able to decode by simply varying the clock rate of the
decoder. I think, as was suggested by someone else, that you would have
to
run the data through a buffer memory, and vary the clocking of the MMU to
arrive at a constant data rate input for the decoder ??

They are "soft sectored", which has been the same for just about every
disk
drive made in the last 30 years. There were hard sectored hard drives,
which disapeared in the 1970's and hard sectored floppies, but only one
or two computers used them in smaller than 8 inch and they did not last
long once soft sectored disks became cheap and common.

They work with a gap between sectors and a header and trailer.

It would be very easy to just "listen" for a header and adjust the clock
speed until it decodes and then read until the correct sector header
starts to come in before doing anything with the data. There is no need
to buffer it and play around with the buffer.

You just have to look for a section of silence (gap) and then a particular
bit pattern (sector header) before you start decoding.

The only cost is time, it may take a revolution or two to "sync up" or not
depending upon how close the clock is to the data.

Geoff.


--
Geoffrey S. Mendelson, Jerusalem, Israel gsm@mendelson.com N3OWJ/4X1GM

Yes, agreed with hard and floppy drives, but I was not aware of there being
any 'sectors' as such - hard or soft - on CD / DVD though. It's been a few
years since I did the courses on CD and DVD, but my poor dim memory seems to
recall that the data format 'belongs to itself' and does not borrow from
other disc technology, hard or floppy. AFAIR, the data is pretty much
continuously recorded - allbeit spread around between frames for error
protection purposes - in constant length frames, and at a linear pitch.
There is no 'silence' to listen for. Because the data is spread around
between frames, it is necessary to have at least a small read buffer anyway,
to allow reassembly of the data and error correction to take place.

Arfa