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Considering Wideband Audio Over The Traditional PSTN

polycomlogoQuestion: What was the first HDVoice product that Polycom offered?

Answer: The Polycom VTX 1000 conference phone.

The thing that makes this fact so curious is not immediately obvious. The VTX 1000 is not a SIP device, nor even IP capable. Like it’s closest relative the Polycom SoundStation 2, it’s designed to connect to a plain old analog phone line (a.k.a. POTS, the PSTN).

Of course, the common wisdom is that you just can’t have wideband telephony over the PSTN. Yet the VTX 1000, circa 2003, delivers wideband conference calls so it seems that assertion is not strictly true. Understanding this is in a little more depth would seem like a good thing. Happily, Polycom published a white paper describing the internals of the VTX 1000.


The VTX 1000 was originally brought to my attention by David Frankel, CEO of the ZipDX wideband conference service. It is essentially a dual-mode device supporting both normal G.711 conference calls like its predecessors, and wideband calls when it determines that the far-end device is another VTX 1000.

The VTX 1000 behaves in some ways much like a FAX machine. Internally, it’s analog line interface is basically a v.34 modem. When it places an outgoing call it sends a sequence of tones, not unlike FAX tones, intended to help the answering devices identify that the calling coming from a VTX 1000.

When a call is answered it listens for these tones. If they are not heard then it knows that it’s just received a normal phone call and it should act appropriately to handle that call as an analog signal source.

If it does hear the calling party declaring itself to be another VTX 1000 then it handles the calls completely differently. Instead of setting up an analog channel it acts like two modems or FAX machines would, and establishes a point-to-point digital channel. This is like setting up an ad hoc network connection over a pair of v.34 modems. You end up with a data channel capable of up to 33 kbps.

With suitable line conditions a pair of VTX 1000 units can pass calls encoded with a wideband codec like G.722.2, which requires from 12.65 kbit/s to 23.85 kbit/s. While the codec used might be based upon recognized telecom standards, the rest of the call setup process can only be viewed a proprietary protocol. Even so, the system delivers real, wideband calls offering frequency response of 80 Hz to 7 KHz….but only between like devices.

Next question: What constitutes “like devices?”

Excellent question! A device like this is meaningless unless there are other like it elsewhere with which it might connect. Of course, other VTX 1000s would work, also the Polycom VSX 8000 Series video conferencing systems, and finally their Vortex EF2280 Integrated Automatic Mic / Matrix Mixer.  This last device being the sort more typically found in larger meeting room installations.

This Post Has 5 Comments
  1. Thanks for exploring my point at this detail. I would like to extend your line of thinking to make a couple of points:

    1. There is no need to stimulate the carriers (wirelines, at least) as advocated by HDConnectNow. Consumer Electronics industry can build interoperable products as you suggest and stimulate the market. After all the market adopted faxes without any government incentives.

    2. More fundamentally, it is time to question the fundamental premise of the famous “Stupid Network” paper and conclude that PSTN IS a stupid network. It is long time that the so called thought leaders of telcom have claimed and been echoing the false assertion.

    1. Aswath,

      While I take a certain part of your points as being sound, I think what the VTX 1000 actually proves is that the approach of tunneling through the PSTN is technically possible, but ineffective. The very fact that many VTX 1000 users never actually realize the benefits of wideband significantly defeats it’s ability to deliver a compelling case for the capability. I think that the VTX 1000 is the test case that proves that this approach won’t ever achieve significant market penetration.

      Wideband audio is in some regards better compared to the STU-III secure phones. To start out it’s a compelling capability to a certain target market based upon their specific needs. But unlike secure phones it has the potential to be more widespread as public experience teaches people to expect more from the telephone.

      The fax machine is a completely different case, delivering a wholly new functionality if it works at all.

      As to whether the PSTN is or isn’t a stupid network, that I cannot say. I’m more inclined to consider such things on the basis of the actions of the carriers than the fundamental nature of the network 😉

  2. Since this was posted we have also learned that the ZipDX bridge supports the Polycom VTX-1000 conference phone. AFIAK, it’s the only service of its kind that does.

    1. I’m not pitching HDVoice. This post was merely to underscore the fact that wideband audio can and has been delivered through the legacy PSTN. That’s without reliance upon IP networking or the SIP protocol.

      It’s also true that to do so has drawbacks.

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