Orban will shortly have a retrofit for 8600s that will implement digital composite, even though, to a certain extent, we've always thought that digital composite was a solution looking for a problem, given the high performance that current analog composite connections provide. Our system is a superset of existing system, and works as follows:
This describes a method of digitizing the entire 99 kHz Composite MPX Baseband signal using a 192 kHz AES3 link.
In early 2013, Nautel introduced an AES3 Composite MPX implementation that uses only the left channel of a 192 kHz AES3 link. This implementation does not allow the entire 99 kHz composite MPX signal to be digitized into a single bitstream. The Nyquist frequency of 192 kHz is 96 kHz, and practical anti-aliasing filters limit the flat passband to a frequency significantly lower than Nyquist. Hence, any subcarriers above about 80 kHz (in particular, 92 kHz SCAs) must be injected and digitized separately.
The Orban implementation uses both the left and right channels to extend the frequency response beyond 99 kHz while maintaining backward compatibility with the left-channel-only implementation. We recommend that exciter manufacturers implement the full AES3 left and right channel implementation because it allows digitization of the entire FM baseband in a straightforward way.
Details:
We extend the original Nautel system by sampling at 384 kHz and multiplexing samples in an even-odd sequence between the left and right channels of a 192 kHz AES3 link. This is equivalent to quadrature sampling at 192 kHz (i.e. sampling a given channel twice at 192 kHz, but with the clock phase shifted by 90 degrees for the sampler that produces the quadrature output). This produces "I" (in-phase) and "Q" (quadrature) signals on the left and right AES3 channels respectively at 192 kHz sample rate. This system has sufficient bandwidth to pass the entire FM baseband (up to 99 kHz) without aliasing
If the input spectrum is limited to 96 kHz or less, either the I or Q channels can be used alone to reconstruct the signal. This makes the system backward compatible with a system that uses only the I signal, like the current Nautel. If there is energy above 96 kHz, reconstructing the original 384 kHz signal’s odd and even samples from the left and right channels will cancel aliasing.
We do not specify any special treatment of AES status bits or user bits. The link uses straightforward 192 kHz stereo AES hardware.
The only unusual requirement is that the frequency response of the left and right channels of the link (including sample rate conversion) must remain flat to Nyquist (96 kHz) if the system is required to carry 92 kHz SCAs. Assuming energy up to 99 kHz in the original baseband, alias energy appears between 93 and 96 kHz in the left and right channels of the link, but the phase relationship of the aliases in the two channels is such that quadrature resampling at the receiver reconstructs any energy above 96 kHz in the baseband and cancels the 93-96 kHz aliases.
This system is unlikely to be used to digitize analog composite. However, for completeness, we need to state that a 384 kHz A/D converter must have a flat passband to 99 kHz. If a given A/D was originally designed for digital audio, it may have a gentle anti-aliasing filter that is only flat to 20 kHz and rolls off gently thereafter, trading ultrasonic frequency response for low ringing in the time domain. So a designer must make sure to choose a digitizer with a “wideband” mode.
What about sample rate conversion in this system? If we sample-rate-convert the I and Q signals with two identical SRCs, phase-locked together, we do not change either the magnitude or phase of the baseband spectra of the I and Q channels, and these remain locked together in time, although both are delayed an equal amount by the filters in the SRCs. This means that the original baseband (with energy above 96 kHz) can be reconstructed by quadrature resampling after a second sample rate conversion at the receiver that restores the original 192 kHz sample rate. However, the intermediate sample rate must not add further aliasing to the signal and must not truncate energy below 96 kHz. In practice, this means that only upsampling is practical. Moreover, the original sample rate must be restored exactly in order to cancel the aliases after quadrature resampling. Hence, sample rate conversion must be done with considerable care and must be synchronous. For upward conversion, the anti-imaging filter that follows the SRC must be flat to 96 kHz. These requirements preclude use of commercial asynchronous SRC chips designed for digital audio. Instead, a synchronous SRC should be implemented in DSP so that the designer can ensure that the bandwidth criteria are satisfied. If a polyphase structure is used in the SRC (as is customary because of its computational efficiency), it should be designed to be flat to 96 kHz.
Because of the complications involved in sample rate conversion, we recommend that the audio path remain at 192 kHz with no sample rate converters inline. The only condition where inline asynchronous SRC is acceptable is if the system is being used in its “downward compatible” mode, where the baseband frequencies must be limited to 96 kHz or less and only the left channel is used. In this case, no aliasing cancellation is required at the receiver, so asynchronous SRC is acceptable.
