Why do I need a good DVD/CD source?


Hi all, newbie question.

If I use the DVD/CD's digital out, either optical or coax, then isn't it true the unit is simply pumping raw 0's and 1's to the receiver?

In theory a $500 unit & a $50 unit's laser would read & send the exact same stream of 0's and 1's, ie whatever is encoded on the disc. Wouldn't it? And all I need is a receiver with a very good decoder.

I understood this part: obviously if I use the analog out of a DVD/CD player, then all the decoding is done in the player & I would need to buy a decent unit.

John A.
It is a good question, Anonymous.

In theory you are quite right; the digital stream of information to the Digital-to-Analogue converter does not care or know anything about sound quality or interference. Each bit (1 or 0), and therefore the whole signal, either gets there or it doesn't.

However, some DVD players cannot go fast enough to collect all the 1s and 0s on the disc. They dip in, and take samples of what is flying by. So the more often they take samples, that is, the higher the "sampling frequency", the closer the sound to the original the engineers decided to burn onto the disc. Most players today can take everything on a CD. But when you get to DVD, the bits are flying past really fast, and it is standard to sample. The Digital-to-Analogue converters (DACs) which do not miss a bit, at DVD frequencies, can be expensive. The quality of the DACs is the single issue in the cost of a player, if there is a single issue. But to keep the DACs happy they also need a good power supply, and the disc has to spin nice and steady, too - otherwise you need massive error correction, like in a modern CD Walkman which stores the stream for long enough to get over being knocked or jogged. But error correction introduces its own problems, and there is a quality issue there, too.

Apart from how frequently the player can sample, there is the question of how big are the samples it can take (16 bit samples required, at least, on CDs; up to 24 bit samples on DVD-audio and DTS surround). Then there is the format in which the samples are read. This is "pulse code modulation", PCM, in DTS and DVD-A, and something else (not as demanding) in Dolby Digital, something else again in SACD. As before, if the machine cannot really take everything on the disc, it "down-samples" - gives the best approximation it can.

So the player can lose some 1s and 0s. It can also add some: error-correction makes a guess at missed bits and adds its own opinion ( or memory) to fill the gap.

If you watch live broadcasts on TV sometimes they break up, and the picture gets divided up into a very small number of very large pixels. The same with sound. It take a lot of sound "pixels" to make the very fast, smooth curve of a sound waveform. If the machine cannot keep up for any reason, the machine can hack it, and give very crude chunks that are a bit like the intended wave shape, but not really. The analogue purists have a good argument, here too: the best digital source just gives the smallest chunks. Real sound is smooth, continuous, analogue. Whether you can tell the difference is another question.

Some of this processing, or hacking, is done at the digital stage. So even the digital audio out is not the same for the same disc playing on different players. Then there is the laser - that is only part of the read-out. It gives out light which is either reflected back from the disc surface (1) or it isn't (0). The refelcted pulse of light is really detected by a photodiode. Photodiodes are pretty fast even when cheap, and they are not usually the issue, but they need to be totally reliable, and each photodiode has to have a good amplifier (the electrical signal from each pulse if very small).

I am not a technical engineer (as you will guess) but this is my understanding. If this is wrong, someone better informed (there are many here) can chip in, and we can learn something.

Is this any help?

Cools thanks again John A. It was I who posted but too lazy to type my name, as it was pretty late at nite. :)

Trying to understand why I should bite the bullet & get a T532. We have a $50 unit that connects directly to our 24" TV via S-video & RCA audio. It does have component out & digital coax so I thought maybe it'll be good enough.

BTW the 24" TV came "free" with our mattress purchase. I'm hoping it'll last a coupla yrs & then it's DLP or LCOS baby!

John A.
By coincidence I have a T532 and the DAC quality is outstanding (better than the T512), and so the sound. The T533, rumoured to be around next February, will have "true" DVD-A. Whether this is a big deal, I am undecided. DTS spec is more or less equal to DVD-A spec, at least for multi-channel, and I cannot imagine better two-channel sound for CDs. You can always get a demo and just listen, you know!
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