A
Alan Rutlidge
Guest
"Richard" <thermof@atps.net> wrote in message
news:1126077325.490966.165880@z14g2000cwz.googlegroups.com...
to 4 Wire Hiybrid Out port) is closer to 7dB loss where the impedance of the
termination connected to the 2 Wire Line port is an infinite mismatch to
that of the impedance of the network connected to the Balance Network port
(ie.e an infinite : 1 ratio). I have measured slightly less (6.7dB) in
proactice on some hybrids, but 7dB is closer to the norm.
The 7dB figure is derived from the fact that a minimum of 3dB loss will
occur between adjacent ports. Add a little extra (say 0.5dB) for
transformer losses and a practical adjacent port loss closely approaches
3.5dB. 3.5dB + 3.5dB = 7dB. A diagram would be easier to illustrate, but
as this is a text only NG, posting a pic is not possible.
wire transmission path, it is not the sole cause. Mismatches due to cable
gauge changes onn a simple 2 wire circuit will produce an echo (reflection)
due to impedance mismatch at the point they join. Echo and signal
reflection are one of the same thing. Just we associate echo with long
return path delays of 35mS or more. It's purely an auditory perception
thing as delays less than 28mS are very hard for the human brain to
recognise.
hybrids if no active transmission components or ADC / DACs were involved.
each hybrid is very close to 3.5dB. As there is a hybrid at each end of the
link, the minimum losses for each direction of transmission would be close
to 7dB.
circuit of t phone would have the transmitter and the receiver in series.
The sidetone level on the receiver would be very high.. The idea of the
ASTIC is to reduce the level of the speech current generated by the
transmitter reaching the receiver, whilst maximising transmitter signal to
line and also maximising the received line incoming speech signal reaching
the receiver. ASTICs are usually single transformer leaky hybrids. Good
examples are to be found in the old Telecom 800 series phones.
the hearing aid version of the 800 series which featured a volume control
where the recall button is usually located.
- In a side note the ASTIC was
high sidetone. This high sidetone level caused people to talk softer
because they believed the other party could hear them okay based purely on
the effect of the local sidetone level. This became an even bigger problem
on long distance trunk calls which suffered significantly more transmission
loss than local calls.
negates the requirement for echo cancellation in either the digital or
analogue domains. This is immediately apparent on facsimile and data calls
through the PSTN (not ISDN) where echo cancellation can't be used. To get
reasonable error free data throughput through the PSTN, the echo performance
of the transmission path must be reasonably good to begin with.
natural sidetone we would normally experience in non telephone coversation.
This means that we need to replace some of that lost natural sidetone with
an equivalent in the telephone. The correct level of sidetone effectively
regulates how loud we speak into the telephone. Ever noticed how spoken but
profoundly deaf people speak - quite often too loud or too soft.
The correct sidetone level in a phone should be approximately 13B below the
speaker's transmission level (local end).
echo cancellation in typical telephone networks. It completely ignores the
issue of VF data (fax and modem calls) through the PSTN and how echo
cancellors are supposed to handle such calls.
Furthermore the document dips to mediocrity with the first sentence on Page
5 which reads " Unfortunately, the hybrid is by nature a leaky device."
What utter crap. I've measured return losses in old 1954 two transformer
hybrids which exceed 60dB! A million to 1 times power isolation is nothing
to be sneezed at. Hybrids, when correctly balanced are supposed to provide
isolation between the 4 wire transmission paths - not leakage.
You've obviously never conducted a Near End return loss measurement on a
customer's telephone line with an EDL423 Network Transmission Quality
Tester. Run a transmission test on a loaded cable (nomimal Z = 1200 ohms)
where it interfaces to a LIB7 LI in an AXE exchange (Zin = 600 ohms fixed).
Where the cable interfaces into the exchange there is a 2 : 1 impedance
mismatch. Even though the mismatch only contributes to 0.5dB additional
forward transmission loss, the return loss at this point is a mere 9.2dB - a
mismatch in anyone's book, with more than 10% of the transmitted signal
reaching this point being reflected back to the source. The only thing
reducing this reflected signal back to the customer is the transmission loss
of the cable itself which by Australian standards must be less than 6.5dB @
820Hz.
Cheers,
Alan
news:1126077325.490966.165880@z14g2000cwz.googlegroups.com...
Trans hybrid loss in a two transformer design hybrid (4 Wire Hybrid In port
to 4 Wire Hiybrid Out port) is closer to 7dB loss where the impedance of the
termination connected to the 2 Wire Line port is an infinite mismatch to
that of the impedance of the network connected to the Balance Network port
(ie.e an infinite : 1 ratio). I have measured slightly less (6.7dB) in
proactice on some hybrids, but 7dB is closer to the norm.
The 7dB figure is derived from the fact that a minimum of 3dB loss will
occur between adjacent ports. Add a little extra (say 0.5dB) for
transformer losses and a practical adjacent port loss closely approaches
3.5dB. 3.5dB + 3.5dB = 7dB. A diagram would be easier to illustrate, but
as this is a text only NG, posting a pic is not possible.
No. Albeit that hybrid mismatch is a major contributor to echo in the 4
wire transmission path, it is not the sole cause. Mismatches due to cable
gauge changes onn a simple 2 wire circuit will produce an echo (reflection)
due to impedance mismatch at the point they join. Echo and signal
reflection are one of the same thing. Just we associate echo with long
return path delays of 35mS or more. It's purely an auditory perception
thing as delays less than 28mS are very hard for the human brain to
recognise.
It would be pointless creating a 4 wire transmission system containing
hybrids if no active transmission components or ADC / DACs were involved.
Agreed. If losses exceed gains the circuit must be unconditionally stable.
Yes. Especially if transformer hybrids are used. The adjacent port loss in
each hybrid is very close to 3.5dB. As there is a hybrid at each end of the
link, the minimum losses for each direction of transmission would be close
to 7dB.
Once again a circuit diagram would assist in the explanation. A very simple
circuit of t phone would have the transmitter and the receiver in series.
The sidetone level on the receiver would be very high.. The idea of the
ASTIC is to reduce the level of the speech current generated by the
transmitter reaching the receiver, whilst maximising transmitter signal to
line and also maximising the received line incoming speech signal reaching
the receiver. ASTICs are usually single transformer leaky hybrids. Good
examples are to be found in the old Telecom 800 series phones.
In the old phones (say 800 series and earlier) there was no gain, except in
the hearing aid version of the 800 series which featured a volume control
where the recall button is usually located.
- In a side note the ASTIC was
As I said previously. Early phones had no ASTIC and sufferred from very
high sidetone. This high sidetone level caused people to talk softer
because they believed the other party could hear them okay based purely on
the effect of the local sidetone level. This became an even bigger problem
on long distance trunk calls which suffered significantly more transmission
loss than local calls.
I beg to differ. Getting impedance matching and hybrid balancing correct
negates the requirement for echo cancellation in either the digital or
analogue domains. This is immediately apparent on facsimile and data calls
through the PSTN (not ISDN) where echo cancellation can't be used. To get
reasonable error free data throughput through the PSTN, the echo performance
of the transmission path must be reasonably good to begin with.
True. When we hold a telephone handset to our head, we cut off some of the
natural sidetone we would normally experience in non telephone coversation.
This means that we need to replace some of that lost natural sidetone with
an equivalent in the telephone. The correct level of sidetone effectively
regulates how loud we speak into the telephone. Ever noticed how spoken but
profoundly deaf people speak - quite often too loud or too soft.
The correct sidetone level in a phone should be approximately 13B below the
speaker's transmission level (local end).
The IEC tutorial takes a very simplisitic approach to the issues surrpunding
echo cancellation in typical telephone networks. It completely ignores the
issue of VF data (fax and modem calls) through the PSTN and how echo
cancellors are supposed to handle such calls.
Furthermore the document dips to mediocrity with the first sentence on Page
5 which reads " Unfortunately, the hybrid is by nature a leaky device."
What utter crap. I've measured return losses in old 1954 two transformer
hybrids which exceed 60dB! A million to 1 times power isolation is nothing
to be sneezed at. Hybrids, when correctly balanced are supposed to provide
isolation between the 4 wire transmission paths - not leakage.
You've obviously never conducted a Near End return loss measurement on a
customer's telephone line with an EDL423 Network Transmission Quality
Tester. Run a transmission test on a loaded cable (nomimal Z = 1200 ohms)
where it interfaces to a LIB7 LI in an AXE exchange (Zin = 600 ohms fixed).
Where the cable interfaces into the exchange there is a 2 : 1 impedance
mismatch. Even though the mismatch only contributes to 0.5dB additional
forward transmission loss, the return loss at this point is a mere 9.2dB - a
mismatch in anyone's book, with more than 10% of the transmitted signal
reaching this point being reflected back to the source. The only thing
reducing this reflected signal back to the customer is the transmission loss
of the cable itself which by Australian standards must be less than 6.5dB @
820Hz.
Cheers,
Alan
