Telephone Anti-Alias Filtering?

[Tim Wescott]

It appears that Google _isn't_ my friend today (or at least not Alta Vista).

I'm looking for information on what's actually done for sampling on the
PSTN.

Anybody know what is current practice for anti-aliasing filters before
digitizing at 8kHz? Anybody know what the practice was back in the day
when it was all built up out of 44mH inductors?

TIA

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Don Bowey]

It appears that Google _isn't_ my friend today (or at least not Alta Vista).

I'm looking for information on what's actually done for sampling on the
PSTN.

Anybody know what is current practice for anti-aliasing filters before
digitizing at 8kHz? Anybody know what the practice was back in the day
when it was all built up out of 44mH inductors?

TIA

I'll get out my Bell reference material and dig up the D4 carrier filter
info. I recall there were numerous updates to the filters as state of the
art improved.

Prior to D3 and D4 carrier, all long haul was on paired cable, coax, and
used analog carrier systems (AM with carrier, without carrier, with separate
sidebands, etc.). What did you want to know about these? Aliasing wasn't an
issue.

Don

 

[Bob]

It appears that Google _isn't_ my friend today (or at least not Alta
Vista).

I'm looking for information on what's actually done for sampling on the
PSTN.

Anybody know what is current practice for anti-aliasing filters before
digitizing at 8kHz? Anybody know what the practice was back in the day
when it was all built up out of 44mH inductors?

TIA

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Tim,

Look at figs 10 and 11 of this datasheet. The anti-aliasing filters are
integrated into the codec -- and have been for a long time.

http://assets.zarlink.com/DS/zarlink_MT8964_NOV_05.pdf

As far as filtering "back in the day", they used passive and active filters
(just like today), but not any switched-capacitor type filters (afaik). The
44mH (and 88mH) inductors you're probably referring to are the ones used in
the so-called 'loading coils' that were placed along long POTS loops to
reduce high-frequency attenuation.

Bob

 

[Nico Coesel]

It appears that Google _isn't_ my friend today (or at least not Alta Vista).

I'm looking for information on what's actually done for sampling on the
PSTN.

Anybody know what is current practice for anti-aliasing filters before
digitizing at 8kHz?

Use an analog 2nd or 3rd order filter (like Sallen-key) at 6kHz.
Oversample at 32kHz and use a 6th order digital low-pass (3.9kHz)
filter, then downsample to 8 kHz. This should do it.

 

[Tim Wescott]

I'll get out my Bell reference material and dig up the D4 carrier filter
info. I recall there were numerous updates to the filters as state of the
art improved.

Prior to D3 and D4 carrier, all long haul was on paired cable, coax, and
used analog carrier systems (AM with carrier, without carrier, with separate
sidebands, etc.). What did you want to know about these? Aliasing wasn't an
issue.

Don

Nothing prior to PCM -- I'm just interested in what is, and was, used to
achieve the voice quality that we enjoy today. I'm working up an
article on sampling because of all the misconceptions I see here and on
the DSP group; I want to use phone filtering as an example but rather
than coming up with some armchair theory I'd like to give examples of
what's actually what.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Don Bowey]

Nothing prior to PCM -- I'm just interested in what is, and was, used to
achieve the voice quality that we enjoy today. I'm working up an
article on sampling because of all the misconceptions I see here and on
the DSP group; I want to use phone filtering as an example but rather
than coming up with some armchair theory I'd like to give examples of
what's actually what.

For the later technologies, the datasheet Bob posted is later than what I
could find in my reference material. The last Bell D4 systems I personally
know of, used active filters on a SIP which started roll off at about 3200
Hz and was down about 70dB by 3400 Hz.

Many of the earlier, analog systems, had a channel response of about 100 to
3500 HZ; many of them used 3700 Hz signaling, which was put out-of-band.
And as I recall, the H-88 loaded cable circuits started rolling off at about
3450Hz.

The improvement in voice quality we enjoy today is, in my opinion, the
elimination of transmission noise and crosstalk. All in all, channel
bandwidth is about the same.

Thinking of noise..... sometimes new technology brings out humorous side
effects. When the O and ON type carriers were fairly new they were very
quiet for the times. Because of this the channel units were changed to add
a noise generator, because the Long Distance Operators didn't hear any noise
when they "plugged into them" and reported them as "out of service."
However with the need for evermore circuits, crosstalk picked up and the
noise generators were turned off.

But we've come a looong ways:

"The first transcontinental telephone service was furnished by loaded 165
(gauge) open wire facilities with repeaters inserted at 500 to 600 mile
intervals."


Don

 

[joseph2k]

Nothing prior to PCM -- I'm just interested in what is, and was, used to
achieve the voice quality that we enjoy today. I'm working up an
article on sampling because of all the misconceptions I see here and on
the DSP group; I want to use phone filtering as an example but rather
than coming up with some armchair theory I'd like to give examples of
what's actually what.

Then start with the CVSD. It is the most misunderstood part of it all. And
of course a tutorial in u-Law and A-Law companders will help.

 

[[email protected]]

Tim,

Look at figs 10 and 11 of this datasheet. The anti-aliasing filters are
integrated into the codec -- and have been for a long time.

http://assets.zarlink.com/DS/zarlink_MT8964_NOV_05.pdf

As far as filtering "back in the day", they used passive and active filters
(just like today), but not any switched-capacitor type filters (afaik). The
44mH (and 88mH) inductors you're probably referring to are the ones used in
the so-called 'loading coils' that were placed along long POTS loops to
reduce high-frequency attenuation.

Bob

The "combo codec" used SCFs, generally designed as ladders. But that
was 80's technology.