Understanding TV systems

Understanding Surround Sound systems

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THX, DTS, and Dolby Digital Briefly Explained

written by Evans A Criswell

The purpose of this page is to explain THX, DTS, and Dolby Digital in the context of movie theatres. These terms are relevant to home theatre systems as well, but there are important differences. For more information about these companies, visit their WWW sites:

First, people tend to put THX, DTS, and Dolby Digital into one "basket", and attempt to compare them. I am going to try to eliminate some of the confusion that results when people do this.

I have heard these types of (incorrect) comments on many occasions:


THX is not a sound format!

First of all, THX is not a sound format. DTS and Dolby Digital are sound formats. By that, I mean that DTS and Dolby Digital are methods of encoding the movie audio digitally and reading it and decoding it in the movie theatre. THX is not a method of encoding sound, but has to do with quality certification. Nearly any THX theatre will use Dolby Digital or DTS as the sound format. SDDS is a sound format from Sony that is also used, but since none of the theatres in Huntsville or Decatur currently use it, I will not go into it.

Well, if THX is not a sound format, then what is it?

THX is a set of quality certification standards developed by Lucasfilm that attempt to ensure that the movie's picture and sound will be reproduced in the theatre in a similar manner to the way it was in the mixing studio. The standards involve speaker positions, auditorium acoustics, and image brightness, among other things. A theatre advertising THX must meet the standards and pay a yearly licensing fee to Lucasfilm.

It is not necessarily true that any given THX theatre is better than any given non-THX theatre. A theatre may be built with exceptional equipment and may have an environment that meets or exceeds THX specifications, but may choose not to spend the money to get tested and licensed. When the THX advertisement is used at a theatre, it simply means that the theatre met the THX specifications when inspected, and Lucasfilm was paid the licensing fee by the theatre. At the time of this writing, the actual THX specifications and the details of recertification are proprietary and are not available to the general public, making it difficult to analyze or comment on THX quality certifications.

Now, let's talk sound formats!

Optical audio and Dolby Stereo

Before talking about the digital sound formats, it is necessary to understand the standard optical audio records that have been used on film for a very long time. The optical audio records work based on how much light passes through the soundtrack as the film moves along. It is read by shining light through the soundtrack and using a solar cell device to convert the light into an electrical audio signal. Originally, the sound was monophonic, but stereophonic sound was made possible by dividing the original soundtrack into two soundtracks and using separate solar cells for each. After stereo was made possible, Dolby developed the matrixed Dolby Stereo format, which was introduced with Star Wars in 1977. The matrixed format encoded 4 channels (left, center, right, and rear) of information into 2, and this format is very similar to the Dolby Surround and Dolby Pro-Logic systems that have been with us for home theatres for quite a while. The theatrical version uses Dolby A or SR instead of Dolby B noise reduction on the rear channel signal.


DTS is a digital sound format, and it stands for Digital Theatre Systems, which is the company that developed the format. DTS was the first digital sound format to be widely used in theatres (starting with Jurassic Park, 1993). DTS is a "5.1" format, meaning that it has 5 full-bandwidth channels (left, center, right, left surround, and right surround) and a low-bandwidth subwoofer channel. DTS, unlike the PCM 2-channel stereo format found on common compact discs, uses a lossy compression method to encode the 5.1 channels of audio into a reasonable size that can be stored on compact disc media and played back from that media in the theatre. Films with DTS sound have a time-code track on the film between the image area and the optical audio records. The information on the time-code track is sent to a computer which then reads the compact disc media to get the compressed DTS sound information. There are three CD-ROM drives, one for a trailer disc, and two for the movie sound. The trailer disc contains the DTS soundtracks for all of the previews being shown in theatres. During a movie, if for any reason the DTS information cannot be read, the standard optical audio records are used as a backup.

Dolby Digital

Dolby Digital is, of course, a digital sound format from Dolby Laboratories. It is a 5.1-channel format, like DTS. The differences in Dolby Digital and DTS are in the encoding method, and the way that the digital information is stored. Dolby Digital compresses the sound information using a different algorithm than DTS. Like DTS, it is a lossy compression method, but the compression ratio is much higher for Dolby Digital, meaning that the Dolby Digital soundtrack requires less data to store than the DTS soundtrack. Dolby Digital soundtracks are placed between the perforations on the film, and are read by a reader that captures the digital information and passes it to the Dolby Digital decoder. Like DTS, the standard optical audio records are used as a backup in case the Dolby Digital audio information cannot be read.

DTS and Dolby Digital are both better in quality than the standard optical audio records with a Dolby Stereo soundtrack. Whether DTS is better than Dolby Digital or Dolby Digital is better than DTS is one of those "religious debates" that I prefer to not get into.

THX is a trademark of Lucasfilm. Dolby Stereo, Dolby Surround, Dolby Pro-Logic, and Dolby Digital are trademarks of Dolby Laboratories. DTS is a trademark of Digital Theatre Systems. SDDS is a trademark of Sony.

Further reading:

An excellent functional description of DVD surround sound systems is found at http://www.timefordvd.com/tutorial/SurroundSound.shtml.

Almost every DVD contains audio in the Dolby Digital (AC-3) format. DTS is an optional audio format that can be added to a disc in addition to Dolby Digital audio. Dolby Digital and DTS can store mono, stereo, and multichannel audio (usually 5.1 channels).  

Every DVD player in the world has an internal Dolby Digital decoder. The built-in 2-channel decoder turns Dolby Digital into stereo audio, which can be fed to almost any type of audio equipment (receiver, TV, boombox, etc.) as a standard analog stereo signal using a pair of stereo audio cables or as a digital PCM audio signal using a coax or optical cable. See 3.2 for more information.

A standard audio mixing technique, called Dolby Surround, "piggybacks" a rear channel and a center channel onto a 2-channel signal. A Dolby Surround signal can be played on any stereo system (or even a mono system), in which case the rear- and center-channel sounds remain mixed in with the left and right channels. When a Dolby Surround signal is played on a multichannel audio system that knows how to handle it, the extra channels are extracted to feed center speakers and rear speakers. The original technique of decoding Dolby Surround, called simply Dolby Surround, extracts only the rear channel. The improved decoding technique, Dolby Pro Logic, also extracts the center channel. A brand new decoding technology, Dolby Pro Logic II, extracts both the center channel and the rear channel and also processes the signals to create more of a 3D audio environment. Dolby Surround is independent of the storage or transmission format. In other words, a 2-channel Dolby Surround signal can be analog audio, broadcast TV audio, digital PCM audio, Dolby Digital, DTS, MP3, audio on a VHS tape, etc.

Unlike Dolby Surround, Dolby Digital encodes each channel independently. Dolby Digital can carry up to 5 channels (left, center, right, left surround, right surround) plus an omnidirectional low-frequency channel. The built-in, 2-channel Dolby Digital decoder in every DVD player handles multichannel audio by downmixing it to two channels using Dolby Surround (see 3.6.2). This allows the analog stereo outputs to be connected to just about anything, including TVs and receivers with Dolby Pro Logic capability. Most DVD players also output the downmixed 2-channel Dolby Surround signal in digital PCM format, which can be connected to a digital audio receiver, most of which do Dolby Pro Logic decoding.

Most DVD players also output the "raw" Dolby Digital signal for connection to a receiver with a built-in Dolby Digital decoder. Some DVD players have built-in multichannel decoders to provide 6 (or 7) analog audio outputs to feed a receiver or amplifier with multichannel analog inputs. See 3.1 for more info.

DTS is handled differently. Many DVD players have a DTS Digital Out feature (also called DTS pass-through), which sends the raw DTS signal to an external receiver with a DTS decoder. A few players have a built-in 2-channel DTS decoder that downmixes to Dolby Surround, just like a 2-channel Dolby Digital decoder. Some players have a built-in multichannel DTS decoder with 6 (or 7) analog outputs. Some DVD players don't recognize DTS tracks at all (see 1.32).

If you have a POS (plain old stereo), a Dolby Surround receiver, or a Dolby Pro Logic receiver, you don't need anything special in the DVD player. Any model will connect to your system. If you have a Dolby Digital receiver, then you need a player with Dolby Digital out (all but the cheapest players have this). If your receiver can also do DTS, you should get a player with DTS Digital Out. The only reason to get a player with 6-channel Dolby Digital or DTS decoder output is if you want use multichannel analog connections to the receiver (see the component analog section of 3.2).


The following details are for audio tracks in DVD-Video. Some DVD manufacturers such as Pioneer are developing audio-only players using the DVD-Video format. Some DVD-Video discs contain mostly audio with only still pictures.

A DVD-Video disc can have up to 8 audio tracks (streams) associated with each video track (or each video angle). Each audio track can be in one of three formats:

Two additional optional formats are provided: DTS and SDDS. Both require the appropriate decoders and are not supported by all players.

The ".1" refers to a low-frequency effects (LFE) channel that connects to a subwoofer. This channel carries an emphasized bass audio signal.

Linear PCM is uncompressed (lossless) digital audio, the same format used on CDs and most studio masters. It can be sampled at 48 or 96 kHz with 16, 20, or 24 bits/sample. (Audio CD is limited to 44.1 kHz at 16 bits.) There can be from 1 to 8 channels. The maximum bit rate is 6.144 Mbps, which limits sample rates and bit sizes when there are 5 or more channels. It's generally felt that the 120 dB dynamic range of 20 bits combined with a frequency response of around 22,000 Hz from 48 kHz sampling is adequate for high-fidelity sound reproduction. However, additional bits and higher sampling rates are useful in audiophile applications, studio work, noise shaping, advanced digital processing, and three-dimensional sound field reproduction. DVD players are required to support all the variations of LPCM, but many subsample 96 kHz down to 48 kHz, and some may not use all 20 or 24 bits. The signal provided on the digital output for external digital-to-analog converters may be limited to less than 96 kHz and less than 24 bits.

Dolby Digital is multi-channel digital audio, using lossy AC-3 coding technology from PCM source with a sample rate of 48 kHz at up to 24 bits. The bitrate is 64 kbps to 448 kbps, with 384 or 448 being the normal rate for 5.1 channels and 192 being the typical rate for stereo (with or without surround encoding). (Most Dolby Digital decoders support up to 640 kbps, so non-standard discs with 640 kbps tracks play on many players.) The channel combinations are (front/surround): 1/0, 1+1/0 (dual mono), 2/0, 3/0, 2/1, 3/1, 2/2, and 3/2. The LFE channel is optional with all 8 combinations. For details see ATSC document A/52 <www.atsc.org/document.html>. Dolby Digital is the format used for audio tracks on almost all DVDs.

MPEG audio is multi-channel digital audio, using lossy compression from original PCM format with sample rate of 48 kHz at 16 or 20 bits. Both MPEG-1 and MPEG-2 formats are supported. The variable bit rate is 32 kbps to 912 kbps, with 384 being the normal average rate. MPEG-1 is limited to 384 kbps. Channel combinations are (front/surround): 1/0, 2/0, 2/1, 2/2, 3/0, 3/1, 3/2, and 5/2. The LFE channel is optional with all combinations. The 7.1 channel format adds left-center and right-center channels, but is rare for home use. MPEG-2 surround channels are in an extension stream matrixed onto the MPEG-1 stereo channels, which makes MPEG-2 audio backwards compatible with MPEG-1 hardware (an MPEG-1 system will only see the two stereo channels.) MPEG Layer 3 (MP3) and MPEG-2 AAC (also known as NBC or unmatrix) are not supported by the DVD-Video standard. MPEG audio is not used much on DVDs, although some inexpensive DVD recording software programs use MPEG audio, even on NTSC discs, which goes against the DVD standard and is not supported by all NTSC players.

DTS (Digital Theater Systems) Digital Surround is an optional multi-channel digital audio format, using lossy compression from PCM at 48 kHz at up to 24 bits. The data rate is from 64 kbps to 1536 kbps, with typical rates of 754.5 and 1509.25 for 5.1 channels and 377 or 754 for 2 channels. (The DTS Coherent Acoustics format supports up to 4096 kbps variable data rate for lossless compression, but this isn't supported by DVD. DVD also does not allow DTS sampling rates other than 48 kHz.). Channel combinations are (front/surround): 1/0, 2/0, 3/0, 2/1, 2/2, 3/2. The LFE channel is optional with all combinations. DTS ES support 6.1 channels in two ways: 1) a Dolby Surround EX compatible matrixed rear center channel, 2) a discrete 7th channel. DTS also has a 7.1-channel mode (8 discrete channels), but no DVDs have used it yet. The 7-channel and 8-channel modes require a new decoder. The DVD standard includes an audio stream format reserved for DTS, but many older players ignore it. The DTS format used on DVDs is different from the one used in theaters (Audio Processing Technology's apt-X, an ADPCM coder, not a psychoacoustic coder). All DVD players can play DTS audio CDs, since the standard PCM stream holds the DTS code. See 1.32 for general DTS information. For more info visit <www.dtstech.com> and read Adam Barratt's article.

SDDS (Sony Dynamic Digital Sound) is an optional multi-channel (5.1 or 7.1) digital audio format, compressed from PCM at 48 kHz. The data rate can go up to 1280 kbps. SDDS is a theatrical film soundtrack format based on the ATRAC compression format that is also used by Minidisc. Sony has not announced any plans to support SDDS on DVD.

THX (Tomlinson Holman Experiment) is not an audio format. It's a certification and quality control program that applies to sound systems and acoustics in theaters, home equipment, and digital mastering processes. The LucasFilm THX Digital Mastering program uses a patented process to track video quality through the multiple video generations needed to make a final format disc or tape, setup of video monitors to ensure that the filmmaker is seeing a precise rendition of what is on tape before approval of the master, and other steps along the way. THX-certified "4.0" amplifiers enhance Dolby Pro Logic in the following ways: a crossover that sends bass from front channels to subwoofer; re-equalization on front channels (to compensate for high-frequency boost in theater mix designed for speakers behind the screen); timbre matching on rear channels; decorrelation of rear channels; a bass curve that emphasizes low frequencies. THX-certified "5.1" amplifiers enhance Dolby Digital and improve on 4.0 in the following ways: rear speakers are full range, so the crossover sends bass from both front and rear to the subwoofer; decorrelation is turned on automatically when rear channels have the same audio, but not during split-surround effects, which don't need to be decorrelated. More info at Home THX Program Overview.

Discs containing 525/60 (NTSC) video must use PCM or Dolby Digital on at least one track. Discs containing 625/50 (PAL/SECAM) video must use PCM or MPEG audio or Dolby Digital on at least one track. Additional tracks may be in any format. A few first-generation players, such as those made by Matsushita, can't output MPEG-2 audio to external decoders.

The original DVD-Video spec required either MPEG audio or PCM on 625/50 (PAL) discs. There was a brief scuffle led by Philips when early discs came out with only two-channel MPEG and multichannel Dolby Digital, but the DVD Forum clarified in May of 1997 that only stereo MPEG audio was mandatory for 625/50 discs. In December 1997 the lack of MPEG-2 encoders (and decoders) was a big enough problem that the spec was revised to allow Dolby Digital audio tracks to be used on 625/50 discs without MPEG audio tracks.

Because of the 4% speedup from 24 fps film to 25 fps PAL display, the audio must be adjusted to match before it is encoded. Unless the audio is digitally processed to shift the pitch back to normal it will be slightly high (about half a semitone).

For stereo output (analog or digital), all players have a built-in 2-channel Dolby Digital decoder that downmixes from 5.1 channels (if present on the disc) to Dolby Surround stereo. That is,  5 channels are phase matrixed into 2 channels to be decoded to 4 channels by a Dolby Pro Logic processor or 5 channels by a Pro Logic II processor. PAL players also have an MPEG or MPEG-2 audio decoder. Both Dolby Digital and MPEG-2 support 2-channel Dolby Surround as the source in cases where the disc producer can't or doesn't want to remix the original onto discrete channels. This means that a DVD labeled as having Dolby Digital sound may only use the L/R channels for surround or "plain" stereo. Even movies with old monophonic soundtracks may use Dolby Digital with only 1 or 2 channels. Some players can optionally downmix to non-surround stereo. If surround audio is important to you, you will hear significantly better results from multichannel discs if you have a Dolby Digital system.

The new Dolby Digital Surround EX format (DD-EX), which adds a rear center channel, is compatible with DVD discs and players, and with existing Dolby Digital decoders. The new DTS-ES Matrix format, which likewise adds a rear center channel, works with existing DTS decoders and with DTS-compatible DVD players. However, for full use of either new format you need a new decoder to extract the rear center channel, which is phase matrixed into the two standard rear channels in the same way Dolby Surround is matrixed into standard stereo channels. Without a new decoder you'll get the same 5.1-channel audio you get now. Because the additional rear channel isn't a full-bandwidth discrete channel, it's appropriate to call the new formats "5.2-channel" digital surround. There is also DTS-ES Discrete, which adds a full-bandwidth discrete rear center channel in an extension stream which is used by DTS ES Discrete decoders but ignored by older DTS decoders. DTS-ES decoders include DTS Neo:6, which is not an encoding format but a matrix decoding process that provides 5 or 6 channels.

The Dolby Digital downmix process does not usually include the LFE channel and may compress the dynamic range in order to improve dialog audibility and keep the sound from becoming "muddy" on average home audio systems. This can result in reduced sound quality on high-end audio systems. The downmix is auditioned when the disc is prepared, and if the result is not acceptable the audio may be tweaked or a separate L/R Dolby Surround track may be added. Experience has shown that minor tweaking is sometimes required to make the dialog more audible within the limited dynamic range of a home stereo system. Some disc producers include a separately mixed stereo track rather than fiddle with the surround mix.

The Dolby Digital dynamic range compression (DRC) feature, often called midnight mode, reduces the difference between loud and soft sounds so that you can turn the volume down to avoid disturbing others yet still hear the detail of quiet passages. Some players have the option to turn off DRC.

Dolby Digital also includes a feature called dialog normalization (DN), which should more accurately be called volume standardization. DN is designed to keep the sound level the same when switching between different sources. This will become more important as additional Dolby Digital sources (digital satellite, DTV, etc) become common. Each Dolby Digital track contains loudness information so that the receiver can automatically adjust the volume, turning it down, for example, on a loud commercial. (Of course the commercial makers can cheat and set an artificially low DN level, causing your receiver to turn up the volume during the commercial.) Turning DN on or off on your receiver has no effect on dynamic range or sound quality; its effect is no different than turning the volume control up or down.

All five DVD-Video audio formats support karaoke mode, which has two channels for stereo (L and R) plus an optional guide melody channel (M) and two optional vocal channels (V1 and V2).

A DVD-5 with only one surround stereo audio stream (at 192 kbps) can hold over 55 hours of audio. A DVD-18 can hold over 200 hours.

For more information about multichannel surround sound, see Bobby Owsinski's FAQ at <www.surroundassociates.com/fqmain.html>.




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