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Optimod-FM 8600 Digital MPX: Overview

Orban’s New Optimod-FM 8600 Digital MPX provides a digital composite output using a 192 kHz AES3 connection. This output appears on a male XLR-type connector on the breakout cable supplied with the Optimod. This output is fully compatible with and interoperable with the de-facto industry standard digital connection being implemented by transmitter manufacturers and others.

A new wordclock/sync reference input appears on a female BNC jack at the end of the breakout cable that connects to the digital composite connector. It accepts a 1x 5V p-p squarewave wordclock signal at 32, 44.1, 48, 88.2, or 96 kHz, or a 10 MHz sinewave or squarewave signal, 0.5 to 5 V peak. 10 MHz is a common output frequency produced by GPS and rubidium frequency standards. You can configure the 8600 Digital MPX to lock its 19 kHz pilot tone and output sample frequency to this input. V2 hardware also provides two new SCA inputs on BNC jacks. These are digitized at 384 kHz sample rate using 16-bit converters and can only be mixed into the digital composite output. (If you need to use both the digital and analog composite outputs, you must split the outputs of your SCA generators with Y cables so that each generator output drives one digitized SCA input and one non-digitized SCA input.)

This package of features can be retrofitted into pre-v2 8600s, but this must be done at the Orban factory.

Several processor manufacturers and transmitter manufacturers have already implemented a system that uses the left channel of the AES3 signal to pass the FM stereo part of the composite baseband, leaving the right channel available. The original 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.

Technology

Orban’s implementation is a 100% backward-compatible superset of the original left-channelonly system and can be used with all such hardware, although its full benefits can only be enjoyed if the hardware receiving the signal is designed to take advantage of it. Orban’s system removes the bandwidth limitation while using the same AES3 192 kHz transport as the original system. We sample at 384 kHz and multiplex the 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 a 192 kHz sample rate. This system has sufficient bandwidth to pass the entire FM baseband (up to 99 kHz) without aliasing. The link uses straightforward 192 kHz stereo AES hardware, and relies on the fact that the AES3 standard allows the left and right channels to be sample-locked and time-synchronized with each other to prevent both phase cancellation in a mono mixdown and widening of the stereo image.

If the input spectrum is limited to 96 kHz or less, either the I or Q channels of the Orban system can be used alone to reconstruct the signal, which is what makes the system backward compatible. 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 in a digital audio receiver designed to take full advantage of Orban’s system.

We do not specify any special treatment of AES status bits or user bits. 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.

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.