CD Transports: Data Drops Etc...

Followup to Aragain:

The CD system depends on the Nyquist Sampling Theorem, which states that a band-limited analog signal (22.05 khz for CD) sampled at at least twice the band limt (44.1 khz for CD) can be exactly reconstructed from the time series of samples.

A tacit assumption behind the Nyquist theorum is that the time interval between the samples be precise and equal to 1/fs, where fs is the sampling frequency (44.1 khz for CD). Any variation in the interval between samples AT THE DAC causes distortion (usually additional non integral high harmonics) in the reconstructed signal.

Thus recovering the data from the disc is only one part of the problem, and the easy part. That's why cheap CD rom drives have no difficulty extracting data from discs in computers. There is no time critical element involved in reading a file.

Audiophile CD transports have no problems getting the data either. It's the timing that causes problems. The clock signal usually starts at the transport (there are exceptions - see the Wadia 270 - 27ix combo). Any tracking problems from a warped disc may affect the stability of the clock at this point.

There are many opportunities for timing errors (jitter) to appear as the signal moves from transport to DAC. Almost all transport/dac seperates communicate using the SPDIF (Sony-Phillips Digital Interface) standard across coax or optical links. SPDIF combines the data stream and the clock together into one signal for transmission on one cable. The receiver at the DAC must extract the clock and data from the SPDIF. This extracted clock often directly becomes the timing reference to the DAC (though some processors go to elaborate lengths to stabilize this clock signal).

The SPDIF signal is itself an analog signal, and is subject to degradations in transit. Thses degradations can alter the waveshape so that the receiver has a hard time decting exactly when a clock transition occurs. This uncertainty is one cause of jitter, and is one reason that digital interconnects sound different.

I'm just scratching the surface here. Going back to T_bone's original note, one could say that the engineering is faulty. For example there are other bus schemes (I2S for example) that transmit clock and data on separate physical lines.

All things being equal (which they never are!) a single box player has an easier time of things since all components are under the designer's control and he/she doesn't have to use SPDIF to transfer data to the dac section.

Remember, that unlike an async serial data transfer between two uarts, the SPDIF signal contains both the data and clock streams intertwined. Unlike a uart, which knows a priori the operating baud rate, the SPDIF receiver must extract the clock from the incoming signal and then use this clock to derive the data stream from the signal.

Any distortions that round off the corners of the analog waveform making up the SPDIF signal cause the receiver to derive a jittery clock signal. In any digital logic, there is a transition zone between the voltages representing the logic states "1" and "0" that is neither state. A rounded off waveform spends more time in this transition zone, thus delaying the detection of logic transitions, thus affecting the derived clock signal.

This effect is not constant.

Improperly terminated coax (RCA plugs instead of BNC connectors, or bad transmitter/receiver design) will have reflections giving rise to standing waves which will cause distortion to the SPDIF signal.

Basic CD player design is still rooted 1983 technology. In those days memory and processor were expensive. I think that one could completely rethink the player design today, using current digital technology and its new price points.

Perhaps the CD drive (at 4x or better) could read ahead, and fill up a circular buffer. The data stream could then be clocked out of the buffer at a constant rate independently of the transport This scheme would even allow for retries of uncorrectable read errors, and would break the direct connection between data timing and transport timing. Of course we'd want to dump SPDIF as a connection mechanism...

I bet it would be easy to make using off-the shelf parts as well.