Why audio out of a DVD player sounds bad


Almost all DVD movies are encoded with Dolby AC3 (Dolby Digital). Some of these AC3 DVD's are 5.1 channel. The 5 is the number of discrete normal channels (LCR and the two surrounds). The .1 is the LFE (Low Frequency Effects) bass channel. The LFE channel is bass effects only and it is sampled at a lower bit rate, it also has +10dB more dynamic range than the front channels.

All DVD players take the 5.1 discrete mix and "downmix" it into a 2 channel Dolby pro-logic mix. This is what comes out of the RCA jacks on the back. They do this for compatibility reasons, since the majority of the world does not yet have an AC3 receiver/preamp/decoder.

Two problems exist.

One is that most DVD players have a sort of dynamic range compression feature that compresses the signal of the front and surround channels. This is similar to a "mid-night movie mode." The exact amount of compression is unknown, it is guessed to be between 10 - 20 dB of dynamic range compression. What this means is that the differences between the quiet parts and the loud parts will not be as great as they would otherwise. Most, if not all DVD players have this "feature". Unfortunately on most DVD players it is not defeatable, I mean you can not turn it off! The Sony DVD players do allow you to disable this so called "feature."

The second problem is that all DVD players that I know of drop the LFE bass channel. This means any information in the LFE channel does not get mixed into the 2 channel downmix. Unfortunately, in action films a great deal of energy exists in the LFE channel. When DVD first came out several people claimed that their LD's had greater and deeper bass. They didn't own AC3 receivers and they were correct. When you understand that the DVD player downmix operation drops the LFE channel this observation makes sense.

Here is why. Most DVD players use the Zoran ZR3852 5.1 AC3 to 2 channel DPL downmixer chip. Two exceptions are the high end 6-chan Panasonic and the Sony DVD players. Well the ZR3852 does the above two things that degrade the audio. It drops the LFE bass channel and it compresses the dynamic range by about 10 - 20 dB. When DVD first arrived, many complained that the Twister DVD THX intro sounds bad when compared to the LD, it has less bass and less dynamic range. In this respect LD audio is superior to DVD audio. Now owning an AC3 receiver, preamp, or decoder bypasses this baddness in the DVD player.

When Dolby Labs was asked why the audio outputs on the DVD players were crippled they said this was done because DVD is targeted at the mass market unlike the niche market LD with its very dynamic PCM mixes. The mass market with their VCR's and whimpy TV speakers can not handle the dynamic range of 5.1 AC3! This is all fine and dandy if you're a HT buf and you own an AC3 receiver/preamp, but if you don't then your bass extension will suffer. Dolby Labs also said this problem would go away when studio's started mixing the audio correctly. This is a nice theoretical solution but it hasn't happened yet (1998).

The audio out of a DVD player has compressed dynamic range and a truncated LFE channel. So what is the solution? Simple, buy a Dolby AC3 receiver/preamp or a DVD player with a 5.1 decoder inside.

Check out my home theatre tips page.