HDVoice In Service Of Online Radio: Part 1

This morning I received a very interesting question from a reader:

I am looking for an IP phone that supports G.722 and has audio inputs / outputs so I can connect it to my mixer. We are trying to connect two studios together for an online radio station. I have yet to find anything other than high end Polycom gear that has something like RCA in/out jacks. Have you by any chance come across anything?

Soljon Zool
Citizen Media Group

While not usually of concern to a small office or home office such issues are routine in broadcasting, which remains how I earn my living. Further, if you’ve read much around here you’ve probably realized that lousy audio quality on radio call-in shows is one of my pet peeves. Even online radio operators should be concerned about audio quality.

Finally, this is a novel application for HDVoice. It’s not something that I’ve done previously, so it caught my interest.

Of course there are dedicated phone interfaces from companies like Gentner Communications. These are the professional way to go about telecom interfaces. Not only are they costly, but they tend to adhere to the legacy PSTN narrowband standards, passing only 250Hz to 3.5kHz. After all, most broadcasters still take calls to air primarily from the PSTN.

Looking beyond commercial interfaces there are probably several ways to achieve what you seek, which is connecting the audio from a wideband conference bridge to your audio mixer. You’ve already clearly stated a preference for HDVoice, so I’ll start by answering the question as asked, and consider how to connect a G.722 capable phone to an audio mixer.

I first thought back across the array of phones that have passed through my doors in recent years. No phone that I know of provide the kind of connections (RCA, 1/4″ TRS or XLR) required to mate to a mixing board. The best that you can do is connect to the phones headset jack.

There are several possible headset connector types on SIP desk phones:

  • RJ-9*
  • Single 2.5mm or 3.5mm 3-conductor (combined mic & headset)
  • dual 3.5mm 3-conductor (mic & headset separate)

*Actually, the term RJ-9 is likely incorrect. According to Wikipaedia:

The 4P4C connector (popularly, though incorrectly, called RJ22, RJ10, or RJ9), is the de facto industry standard for wired telephone handsets. It is used to provide connection from the base of the telephone to the handset.

Whatever it’s called this connector is less than convenient, although it does carry the signals that you need. Fortunately, ZoomSwitch makes a little adapter cable called the ZMS15 adapter cable that adapts RJ-9 (aka 4P4C) to a pair of 3.5mm jacks (mic & headset) more typical of a PC sound card. This adapter could easily be made in the field for a few dollars if you can find the parts.

The headset output is an unbalanced audio output. If you’re very concerned about signal integrity you use an active interface to change the unbalanced signal from the phone to balanced signal for the mixer input. These run the gamut from $50 to $500. Such an interface device is probably only required of you have some long cable lengths involved in your production circumstance.

Some phones, like the Cisco SPA-9xx series, provide a single 3.5mm 3 conductor headset jack which, being more common, is a little easier to deal with. A very few phones provide a pair of 3.5mm 3 conductor jacks, mimicking the mic and headset jacks common to a PC sound card.

From personal experience I can say that the older and now discontinued snom 200 has just such a connector compliment, as does the Gigaset DE380IP-R. (pictured below)

It happens that I have a sample of the Gigaset DE380 on-hand. The following picture is the rear panel of the DE380 showing the pair of 3.5mm audio jacks.

The availability of a suitable phone made it a simple matter to I stage a brief experiment. I used an adapter cable (3.5mm 3 conductor to 2x RCA) to connect the DE380 headset output to the line-level input of my Spirit Notepad audio mixer.

Given that I had a suitable phone and the right cable readily available this little experiment took only a few minutes to setup. The pair work well together. I can hear the audio from a call through my office sound system in perfect clarity.

Given a suitable adapter I believe that just about any decent phone could be connected to a line input on the mixer in this fashion. There are G.722 capable phones from many manufacturers including; Aastra, Avaya, Cisco, Gigaset, Grandstream, Mitel, Panasonic, Polycom, snom and Yealink.

My theory is that all is required is to get the audio stream from the conference bridge into the mixer. I presume that the host could be using another phone to join the conference himself. Thus there’s very possibly no need to worry about connecting anything to the microphone jack on the phone.

It would be best if the host could use another phone to join the conference. Ideally, he’d wear a good headset so as to provide optimum audio into the conference bridge. The second phone then just provides an audio feed from the conference bridge into the mixing board.

One of the reasons that you’ve not heard me make much mention of the Gigaset DE380 is simply that it’s not one of my more favorite phones. I think that for the price the Polycom SoundPoint IP335 is a better option.

There’s a second approach that I’d like to describe, but as this getting long enough I will leave that for another post.

  • FYI:
    Another G.722 phone with built-in stereo line-level outputs:
    http://www.grandstream.com/products/gxv_series_phone/gxv3140/gxv3140.html

    • The Grandstream GXV-3175 also has readily usable analog audio connections.

  • Even though this is an older article. I found this VERY helpful. I was looking at a solution for a local radio station in which we are the ISP and now phone provider. The question was presented, How do we get the studio phone to air a phone call from a remote site where they call in through a cellular connection and broadcast it live? I think this may have answered this question. My initial thoughts were down the same line; however, it is a very nice thought about putting it into the conference bridge! 

    • mjgraves

      Glad you found it helpful. Now that the Opus codec is starting to see some use it’s possible to get some true, production-quality audio over reasonably low-bandwidth IP-based links. Tools like Freeswitch make it possible at low cost, if a little on the DIY side.