Telephone Anti-Alias Filtering?

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

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.

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

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.

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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.

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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

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.

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