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Digital inputs

Adding digital inputs seems like an easy idea. But the reality is unfortunately different.

The problem is that when you are playing an optical disc, there is a master audio clock that controls everything, particularly the conversion rate of the DAC chip itself. There is a buffer to hold audio data in the player, and if the buffer gets too full, the control circuitry slows the disc down. Conversely if the buffer becomes too empty, the disc is sped up.

The key thing is that the master clock is in charge of everything. It is a fixed-frequency device, and can be made with exceptionally low jitter.

But adding a digital input creates a complete can of worms. The problem is that the master clock has now been moved to the transport. The DAC must now adapt to, and follow the transport. This means that the clock in the DAC must be variable in frequency. A variable frequency clock always has higher jitter than a fixed frequency clock.

The problem is further complicated by the fact that the only existing format for the transfer of digital audio data (S/PDIF) is basically broken. Trying to recover the clock from the signal transmitted from the transport is a tough nut to crack. Basically, there are no clever trick here -- the only way to get better performance is to throw money at the problem.

So if you want a digital input, the choice is add a cheap one for several hundreds of dollars (but that will sound quite mediocre) or add an expensive one that will sound good for several thousands of dollars.

The first way hurts the reputation of the player (and the company), while the second way will result in sharply reduced sales, and for a feature that probably less than 10% of the customers would ever use.

So that is why you rarely see a digital input on a player.

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