Super Audio CD: The Rich Report Page 4

Why am I skeptical about the "simple signal path" explanation? In addition to the practical problems of converting an SACD datastream directly to analog that I discussed above, the proposed simplified signal path assumes no signal processing between the digital tape recorder and the SACD mastering process. Even something like mixing a multitrack recording—mixing can occur directly on a delta-sigma stream—results in a multibit stream, even if the mix occurs one-to-one. While it is possible to equalize and even add reverb in the delta-sigma domain, these again result in multibit streams. Once we have a multibit stream, we must convert it back into a 1-bit stream using a digital process similar to noise-shaping. These operations will have to be repeated many times as cascades of EQ and mixing operations occur.

Yes, pro audio companies are introducing the EQ and mixing tools to allow engineers to work directly on material recorded in DSD. But it is fair to point out that, unless studios are completely redesigned, it is highly likely that SACD signals will be converted to PCM signals to allow mixing, equalization, editing, etc., using current digital equipment. And at the consumer's end, it is very difficult to implement digital room-correction systems and digital loudspeaker crossovers in the SACD domain. Even if this becomes possible, we will once again be faced with a multibit stream that must be processed back to a 1-bit stream.

Again, any processing of the 1-bit stream on the user's end assumes that it will be available at the output of the SACD player. For copyright reasons, this is something that might never be made available.

What is clear is that the Sony/Philips Super Audio CD system does not perform to the same level as the best LPCM converters now available. The best ADCs' delta-sigma modulators, for example, produce four times the data per second as DSD, which means a lower noise floor and lower levels of distortion than can be produced with DSD (assuming the same loop-filter order).

In addition, even though current chips can perform, at best, only at a true 19-bit level, DVD-A offers a capacity of 24 bits. We can expect the number of bits produced by the finest ADCs, and the sampling rates at which they operate, to continue to increase in the future as chip designers attempt to come as close as possible to the 24-bit performance level. Advances in silicon-chip manufacturing that allow the development of very fast microprocessors—including copper interconnects, trench isolation, and high-energy implantation—can be applied to speed the sampling rate of ADCs as well as allow them to encode more bits per sample. In addition, the introduction of very-high-speed BiCMOS-process technologies developed for radio-frequency systems, including new silicon germanium bipolar devices, will allow the analog circuits in delta-sigma-based ADCs to work well at these higher sampling rates.

And keep this in mind: Burr-Brown already produces DACs that do 96kHz sampling with 24-bit resolution—resolution, not the levels of noise or distortion—and which use no feedback. Using the very-high-speed converters designed for cellular radio applications, it is already possible to do A/D conversion at the 18-bit level without significant feedback. Would such converters give better sound than the delta-sigma ADCs used to make current 24/96 recordings? (And these delta-sigma ADCs, oversampling at twice DSD's rate and using at least three-code quantizers within their delta-sigma loops, are already better than DSD's 64Fs 1-bit converters.) We don't know the answers to these questions.

While CD was introduced as "perfect sound forever," it had only two channels of output, which put the lie to that statement right away—clearly, multichannel systems were required to create the sound of a good concert hall. Both SACD and DVD-A offer multiple channels; but with DVD-A, the space available for experimentation and improvements is almost infinite. With Super Audio CD, we are stuck with a system that cannot be improved. It is likely that SACD was brought to market not as a way of bringing audiophiles closer to the music, but of making digital audio more difficult to copy.

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