Help me understand sound encoding formats!!

Factual Questions
1

I am confused. Nothing new there, but this time it’s about something specific - sound formats. Can someone please help me to understand what, exactly, the differences are between Dolby Digital, Dolby Digital EX, DTS 5.1, DTS-ES 6.1 (matrix/discrete), Dolby Pro Logic II, Logic7 and so on? Who set the formats, what is the approximate chronology of their development, and what is best (an ordered list would be useful). If you connect a subwoofer to the LFE channel on a HT system, will that play the low frequencies of any music or other programming, or only the information specifically encoded for the LFE channel? Also, is THX a format specification, or just a quality control standard?

Any knowledgable explanations would be appreciated.

-FK

2

Dolby Labs is the driving force in theater and DVD audio encoding. They are responsible for the Dolby Surround, Dolby Digital, Dolby Digital EX, and Dolby Pro Logic / Pro Logic II encoding technologies. Dolby Pro Logic/II is a matrix coding technique, where multiple channels of audio can be coded into a single channel, then decoded at playback. The coding process causes significant quality loss for the coded channels, but it’s better than not having those channels at all. The “original” surround sound format was Dolby Surround, which was a 4.0 surround format, where the rear channels were matrix coded into the front channels, so in reality only stereo audio had to be recorded.

Dolby Digital/EX audio you hear in the theaters and the Dolby Surround and Dolby Digital/EX audio you hear on a DVD is stored in the Dolby AC3 compression format. In theaters its stored at a bitrate of 640kbps, on a DVD its usually at 448kbps. Dolby Digital you get on a DVD is usually three front channels and two surround channels, plus a Low Frequency Effects channel, making it 5.1 surround. Dolby Digital EX matrix codes two additional rear channels into the surround channels, thus producing 7.1 surround when properly decoded. If an LFE is present, there should be no low frequency information in the other channels. If you play a Dolby Digital stream with an LFE channel on a system without an LFE channel, the LFE information should be mixed into the left and right channels. When playing audio without an LFE channel on a system with one, a process called bass redirection should redirect low frequency sound to the subwoofer. YMMV, depending on your speaker system.

DTS is an alternate audio encoding format from Dolby Digital AC3. It uses much higher bitrate, between 700-1500kbps, as I recall. Most people agree that this allows DTS to have substantially higher quality than Dolby Digital. DTS stores each channel seperately, so a 5.1 surround DTS file will have, in essence, six mono channels encoded seperately. Dolby Digital AC3 uses channel coupling, which reduces the space required to hold an audio stream, but may negatively effect the channel seperation and phase. This is the major reason why DTS tends to have better audio quality. I’m not sure how DTS ES stores the extra channels of audio data, but I believe it codes it in a similar manner to Dolby Digital EX, by embedding it in some existing channels. I may be wrong, however.

THX is, as you guessed, simply a quality control standard. They certify speakers and sound systems as meeting their quality specifications.

I’ve never heard of Logic 7, sorry.

If you need any further clarification, feel free to ask.

3

If you connect a subwoofer to the LFE channel on a HT system, will that play the low frequencies of any music or other programming, or only the information specifically encoded for the LFE channel?

I wondered about this too, and for the most part on today’s HT receievers, the answer is yes. It amounts to the receiver having several different “modes” depending on the source. If it’s receiving a 5.1 signal from a DVD player, then of course the LFE channel will be playing that .1 portion. Now if you’re playing a CD, you’re just handing the receiver a stereo signal, two channels. Usually in the setup menus of the receiver, you tell it what speakers are connected, and it takes this into consideration. So if you have a subwoofer attached, it’s going to use it, and may even provide a crossover to cut the lows going to your main speakers. From here it’s a hop, skip, and jump to various Digital Signal Processing goodness, such as providing different listening fields (Hall, Theater, etc) artificially produced by delaying the sound to your surround speakers when listening to a regular stereo source. In a nutshell (too late I guess) most HT receivers will use all of your connected speakers, including the subwoofer, when playing a stereo source.

4

*If you connect a subwoofer to the LFE channel on a HT system, will that play the low frequencies of any music or other programming, or only the information specifically encoded for the LFE channel? *

Oops, it occurred to me I did not actually answer your question. If you are listening to a signal with discrete channels (Dolby Digtal 5.1, DTS, etc) the LFE channel is only going to play what is on that channel. I don’t think it makes any decisions itself, this was all decided when the movie was mixed. That being said, the LFE channel is most definately used for music, explosions, and whatever other movie sounds require low frequencies. I would imagine when movie sound engineers are mixing the sound, they pay close attention to center, main, and surround channels, and just let the LFE channel handle all the lows, since it is nondirectional and is the only speaker capable of those frequencies anyway.

5

It occurs to me that I should probably better explain channel coupling, since it is the main cause of the quality difference between DTS and Dolby Digital AC3. I’ll use the example of a stereo audio stream with a bitrate of 192kbps to keep things simple. In a “pure” stereo stream without channel coupling (DTS), you’d have two mono channels of 96kbps each. In a file using channel coupling (AC3), you might have the left channel encoded at 128kbps, and then a 64kbps stream telling how to reconstruct the right channel from the left channel.

Channel coupling allows the audio to be coded much more efficiently, squeezing every last drop of quality out of the available bitrate. However, channel coupling will negatively affect the stereo seperation and phase of the audio. DTS simply devotes a huge amount of space to the audio (768kbps or 1536kbps, or 128kbps or 256kbps per channel), making high coding efficiency, and thus channel coupling, unnecessary. This allows it to offer better channel seperation and overall quality than Dolby Digital AC3, as Dolby Digital has to use channel coupling to cram all of the audio into 448kbps, or roughly 75kbps per channel.

Overall, IMHO the difference between DTS and Dolby Digital AC3 isn’t huge, but DTS does sound better.