Field Report: Comrex Matrix


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Transmitting audio via telephone circuits has been a necessity in radio for quite some time. In the beginning, radio used plain old telephone service and a plain old telephone. It was fast, cheap, easy, and sounded awful. The next step up was conditioned, or equalized, lines. This worked fine while the phone company had technicians that knew how to set them up and they cost only $125 to install. If you can even get one today, install prices are about $1,500 to $2,500 for a single line. Not very cost effective for a quick remote from the local Donut Hut.

Our stations, WBQB and WFVA, are remote intensive, especially during the spring and summer. Although we have a well-equipped broadcast van, complete with pneumatic mast and remote pickup equipment (RPU), there are times and situations where RF remote gear is impractical. RPU equipment is good for short-range work, but interference is a constant problem, especially in metro areas. It also requires a crew with the technical savvy to operate the equipment. Plus, it's often tough to get a clean signal out of malls and other large venues.

Performance at a glance

  • Uses standard POTS lines

  • Full-duplex 15kHz audio

  • Easy to learn and use

  • Built-in mixer

  • Up to 7 hours operation on battery

  • Clear, concise manual

With our stations' desire to do remotes beyond the range of our current RPU system, I wanted a device that could be setup easily, give us excellent audio quality, and minimize operational cost. After some investigating, I decided to try one of the POTS codecs. Our FM station had a contest coming up that required a live remote broadcast for five straight days, twenty-four hours per day. I felt that a week-long remote with limited technical staff on hand would be a trial by fire for any device. I contacted Comrex and described my situation. The company was very receptive to the idea of putting the Matrix to the test.

Data Rate (kb/s)

Vector Mode

Hotline Mode

9.6

5kHz

n/a

12

6kHz

4.5kHz

14.4

7kHz

5.4kHz

16.8

7kHz

5.6kHz

19.2

11kHz

5.8kHz

21.6

12kHz

6.5kHz

24

15kHz

7kHz

26.4

15kHz

8kHz

28.8

15kHz

8.6kHz

31.2

15kHz

9.3kHz

33.6

15kHz

10kHz

The various audio bandwidths based on operating modes and connection rates.

The Comrex Matrix is a single-line POTS codec that provides 15kHz, full-duplex (simultaneous send and receive) audio at connect rates of 24kb/s and above. Below these rates, the audio response ranges from 12.7kHz to 4.7kHz at connect rates of 21.6kb/s to 9.6kb/s. A Voice Mode allows 7kHz audio with a 300b/s ancillary data channel and extra forward error correction. The Matrix can also communicate with the other Comrex codecs, the Hotline and Vector. The Matrix also has provisions for plug-in modules to be added later. Two such modules enable the Matrix to work with ISDN lines, and GSM wireless services.

Get up and go

The Matrix is easy to set up. The studio end requires a telephone line and a connection to the console or switcher. A return audio path should be connected as well. This return feed should be a mix-minus of the remote signal, as the encoding/decoding and transmission process takes about 100ms. Off-air monitoring at the remote site will be disconcerting, especially to the talent, because of the audio delay.

At the remote end, the portable unit has provisions for mixing two audio sources. One is switchable between mic and line, the other is mic only. There is a line out for connection to a PA or personal monitor device, plus a headphone jack. This is an adjustable combination of the mix-minus feed provided by the studio and the local audio sources, so those at the remote site can hear themselves in real time. A tally closure can be triggered by the user at either end, and can be used for cueing or remotely starting an event.

The Matrix also has a unique feature called Store and Forward, which allows you to record an audio feed up to nearly ten minutes and then send it via a low-speed circuit that won't allow real-time codec use.

Dialing from either end is possible with the number being held in the #1 memory position for easy redial. The well-written manual also suggests that if your connection is enabled for more than a couple of hours, you should renegotiate it to allow the modems to adapt to changing line characteristics. This is good advice, because if the unit at either end detects a change, it will automatically activate renegotiation, which shuts down the audio for about 10 seconds. According to Murphy's Law, chances are good that this will happen in the middle of a break during a busy morning show. I put together a quick one sheet for the air staff describing the operation of the Matrix in brief, and how to renegotiate every two hours with the one button command. Since doing this, we have had no further unexpected outages.

Data Rate (kb/s)

G.722

Turbo G.722

Layer III

56/64

7.5kHz

n/a

15kHz

112/128

n/a

15kHz

n/a

Coding Delay

6ms

6ms

300ms

Comparison of audio coding delays for encoding algorithms based on connection rates and frequency response.

During initial setup, I added two RF filters to the studio phone line, because the close proximity of our AM stations' transmitter prevented the Matrix from working properly. Although the manual does not recommend connection through a PBX or phone system switch, I tested the portable end through our phone system. Surprisingly, I routinely obtained a solid 14.4kb/s connection, with occasional 19.2kb/s connections. These results were good enough to have been used on air if needed, but other situations could be different.

During the continuous 100-hour remote broadcast, we experienced no problems with the Matrix. Everyone was very impressed with the audio quality, especially considering it uses a regular phone line. The return station audio especially sounded good. Plus, it was very easy to use. The proof of our satisfaction is that we've already ordered our own Matrix system from Comrex.

John Diamantis is chief engineer of WBQB and WFVA in Fredericksburg, VA.

Editor's note: Field Reports are an exclusive BE Radio feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.

It is the responsibility of BE Radio to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by BE Radio.

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