Noise is the enemy of good sound in any stereo component. There are many sources for noise, and there are just as many ways around the problem. One of the common ways to control noise in a
digital playback system is to isolate the loud, clunky and vibrating parts that spin the disc from the delicate high frequency digital and
analog circuitry.
Doing so yeilds a digital transport and a
DAC in separate boxes. Thus they are often called "separates." The digital transport is the hardware that physically extracts the data from the disc as it spins. The
digital data from the laser is sent through an
interconnect or
proprietary umbilicus to the
DAC. The DAC converts that data into the analog signal that the amplifier sends to the speakers.
By putting the disc/tray/
laser assembly in a separate box, you can keep it vibrationally damped and give it it's own power supply. These things will keep it happy and help it provide the cleanest stream of bits to the DAC. But the DAC benefits far more greatly from the separation. The minute digital and then analog signals that it is processing are very susceptible to
interference, not only from the
EM fields that the transport generates, but from the noise that those pieces of equipment often feed back into the power supply, which will then contaminate the DAC circuitry.
Most
CD,
DVD-V,
DVD-A and
SACD players are constructed as a single stereo component. However, many of them have digital datastream outputs in several formats, the most common being coax and
TosLink. See my writeup on
interconnects for more detail. This allows you to purchase a DAC and connect it to your CD/DVD player, most often yielding significantly improved sound. Connecting the DAC will make your CD/DVD player into a digital transport. Unless you spent several thousand on your CD/DVD player, chances are that any outboard DAC you use will be of higher quality than the one built into the unit. These converters can cost from as little as a few hundred dollars to over $50,000. Not only will the DAC improve the sound, but it will also allow you to add features that the transport my have lacked, such as
HDCD Decoding or
upsampling.
One problem that is exacerbated by separating the components thusly is
jitter. When the circuitry generating the digital data is physically separated from the circuitry that must decode the data, they have to use separate clock oscillators, these can often end up out of synch which results in data errors, where a 1 is read as a 0, or vice versa. These errors, though
miniscule, produce audible degradation in the sound, like a hazy smoke.
If the jitter in a system is bad, the solution is to install a jitter reduction box along the digital interconnect, this unit should re-create the original clock out of the datastream and send out cleaned-up, reclocked stream. Jitter is only a problem on very high end stereo systems, for most of us, there is so much other noise and
distortion in the system that the jitter is entirely obscured.