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Can I distribute IP Audio over a SCA channel with an Instreamer ?

We have been asked if an SCA channel could be used to route traffic generated by an Instreamer via an FM station.

Unfortunately, the quick, general answer is NO, this is impossible .. and here is the technical explanation:

An SCA channel, which is a 5kHz bandwidth sub-carrier on an analog FM station transmitter, allows transport of an audio signal in addition to the "standard" stereo signal which is received by the standard FM receivers. Such an SCA channel uses an otherwised unused part of the FM station bandwidth. Details you can google on the internet.
Traditionally, bandwidth-limited audio transmission technologies (like the plain old analog telephone system) have been used to carry digital data by using modems to convert serial data streams into audible signals which can be transmitted over a (voice) audio channel.
A 5kHz bandwidth sounds like it could be used like a phone line - right ?

Well, principially, that is possible. You CAN use a modem of some sort to encode a serial data stream and transmit it via an SCA channel, this has been done since at least 25 years now. BUT: the bitrate you can achieve with a one-way connection is very limited (according to Shannon's theorems, it is based on bandwidth and snr, a normal phone line can not carry more than roughly 30kbps in the optimum case).
Any promises of "higher" bitrates (like 33.6k, 56k modems etc) use compression techniques and typically need 2-way communications. If you would want to transmit an MP3 encoded stream, for example, you would not be able to compress much on the data stream, which means these modems would not deliver 56k ..

In any case, to carry IP (network) data over the SCA channel, you can NOT directly connect the network connector to the SCA channel encoder ! You will need a router and a modem to connect in between. The router serialized the network packets (arriving with 10Mbps or 100MBps speed), and sends them via a (slower) serial link, into the modem, which converts the serial stream to an audible signal, which then can be put into the SCA channel. The receiver end uses the inverse chain, a modem receives the SCA audio channel, converts to a serial stream, which is then put into a router to recreate IP/network traffic.

All good - so why can't you use an Instreamer and Exstreamer with this ?
Well - the achievable bitrates over the SCA channel will be in the range of 9600 baud to maybe 19200 or (i guess absolute max) 28800 kbps. That is enough to get contact closure information or maybe a short text (title information etc) over the SCA channel, even if encapsulated into IP (which adds a lot of overhead to the raw data !), but for an IP Audio stream, encoded by an Instreamer, which requires at least 40kbps plus overhead, this is much too little.
Satellites provide much higher bitrates for aux channels, so a similar application might work very well with a satellite system (but i have not done any research into this).

So - bottom line, if you want to carry anything via IP and distribute via an FM SCA channel, you will face these limitations:

  • very low achievable bitrate
  • one-way connection only (unidirectional from transmitter to receivers), no retries etc
  • suitable for contact closures or small text messages
  • not suitable for an IP Audio stream using MP3 or PCM or other "standard" codecs.

There are specialized encoders/codecs for low quality, very low bandwidth algorithms (military stuff uses down to 1200bps, cellphones use often 9600baud), so you could probably encode an audio stream with very low quality to send it over an IP link using 28.800 baud modems, but definitely with inferior quality than feeding the audio signal itself.

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