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HDVoice In Service Of Online Radio: Part 2

Michael Graves

In part 1 I addressed Soljon’s question about how to physically connect a G.722 capable SIP phone to a traditional audio mixer for use in an online radio project.

I understand and appreciate the intention to use a phone as the audio interface device. Phones are effectively appliances, offering excellent audio quality combined with simplicity of operation and high reliability.

This very logic leads me to use my Polycom IP650 in some unusual ways. For example, when I occasionally guest host the VUC calls I will call the ZipDX wideband conference bridge on one line, then call the Talkshoe G.711 bridge on a second line and perform an on-phone conference to connect the two bridges. Finally I engage the call recording function on the IP650 to give me an uncompressed WAV recording of the entire call.

The uncompressed recording of the call is more convenient for post-production purposes. I usually prefer to work in the uncompressed domain, then create a final MP3 for release as podcast as the last step in the post-production process.

In these instances I don’t join the VUC call using the Polycom phone. I usually join the call using Eyebeam and a cordless headset. This gives me some independence. I can drop off the call and know that the bridges are still joined and the local recording is continuing.

For Soljon’s situation soft phones offer some flexibility that a hard phone may not deliver. There’s a second wholly different approach that you might take based upon using a soft phone and connecting the host computer to your mixer.

To do this you’ll need some kind of audio capability on the host computer. There are myriad ways to get audio in/out of a computer, some cheap and some very costly. If the host computer doesn’t have audio capability you can always add it by way of a $10 USB sound adapter (pictured above.) I’ve frequently used such simple hardware to record screencast tutorials with voice annotations. They can sound surprisingly good, certainly as good as a G.722 encoded phone call.

The weakest aspect of such cheap audio interfaces is the microphone preamp. However, on our example application we don’t even need the mic preamp, so the cheapo adapter will work just fine.

Unlike in earlier years, there are now numerous soft phones that support G.722 based wideband audio. Here’s a list just off the top of my head:

While there have been issues with faulty or incompatible G.722 implementations these are becoming less common as over time there is more and more real-world G.722-based interop happening. Even so, you should test the soft phone you select to ensure that it works well with your chosen conference bridge and hardware phones.

Given that a soft phone will use whatever audio interface you have on the host platform it should be no more difficult to connect it to the mixer than a hard phone. Most likely a 3.5mm to RCA or 1/4″ TRS cable is all you need.

Soft phones tend to be relatively light applications which means that you can pick a low-cost host platform like a netbook or net-top and treat it like an appliance. You may even select a thin client or embedded PC as I described recently when discussing DIY Asterisk Appliances.

I have used Eyebeam and PhonerLite on my HP 2140 and 5102 netbooks and not had a problem as long as that was the only app that I was running at the time.

Netbooks are nice as platform because they are cheap, small and include a keyboard, monitor and mouse built-in. That means that once you have the call up and running you can place them safely out of the way and go about the business of the call.

As I described previously, the most ideal production circumstance may not be the obvious one with respect to the host. You really want to the host to be primarily engaged in the conference call using some kind of phone. Then take an audio feed from the conference call into the production mixer.

This simplifies that audio handling process considerably since you don’t need to feed the output of the mixer into the conference call. This reduces the likelihood of echo and feedback on the call, albeit at the cost of degrading the hosts audio to that of the conference call itself.

The conference bridge has the ability to send each participant what is known in music as a “mix minus” feed. That is, each person hears others on the call, but not themselves. When you try to include the production audio mixer into the call in both directions (sending and receiving audio) this becomes much more complicated to do.

If the soft phone is merely being used to pass the call audio into the production path then you can simply mute the mic on the soft phone secure in the knowledge that it will not introduce noise into the call or the program audio. Many soft phones will also record the call giving you a backup local recording for possible use in post-production.

At every turn down this path I seem to discover slightly different and potentially advantageous way of doing things. So, believe it to not, there will be a part three to this little series. In part three I will look at some unusual ways to leverage soft phones that you probably would not have considered.

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