Dolby Pro Logic, Dolby Digital, DTS and THX Surround EX Explained
Mar 29 '01
The Bottom Line The format you choose mostly depends on the amount of money you are willing to spend, equipment you already have and the level of performance you are willing to accept.
Here is a quick rundown on the features of popular surround sound formats: Dolby Pro Logic, Dolby Digital, DTS and THX Surround EX.
1. Dolby Pro Logic is the simplest, cheapest format. It has 4 channels that are compressed into two analog channels, so you can record it on your stereo VCR and it can also broadcasted even over analog radio or cable TV. Another advantage – Dolby Pro Logic can be easily downconverted from a Dolby Digital signal.
These are 2 discrete and 2 matrixed channels:
2 discrete, full-bandwidth channels (left front, right front),
1 matrixed full-bandwidth center channel,
1 matrixed, limited-width channel that goes to 2 surround speakers.
The Dolby Pro Logic encoder takes 4 channels (L, R, C, S) and creates two output channels (Lt, Rt). L goes to Lt and R goes to Rt without modifications, C, after a 3dB level reduction, gets divided equally between Lt and Rt. The bandwidth of S gets limited to 100Hz - 7 kHz range, it is encoded with a form of Dolby B noise reduction. After that it gets divided between Lt and Rt but with a +90 and -90 degree phase shifts.
During the playback, the surround is extracted by analyzing the difference between Lt and Rt.
Disadvantages are: both surround speakers produce the same sound from the same channel (mono sound) and are limited in bandwidth. Additionally, there is no discrete subwoofer channel specified in the standard, so subwoofer channel can extracted from other channels rather than specified explicitly.
Sources for Dolby Pro Logic include TV shows and movies, videotapes, other analog sources and downconverted signal from Dolby Digital (DVD for example).
2. Dolby Digital is a 6-channel format, referred to as “5.1-channel”. All 6 channels are discrete, 5 channels are full-bandwidth, “.1” refers to subwoofer, which supplies Low Frequency Effects (LFE).
To compress data with low loss in the sound quality, audio coding, known as "perceptual coding" is used. Dolby Digital uses a third generation audio coding algorithm (AC-3).
The channels are: front left, front right, center, surround left, surround right and subwoofer.
Advantages over Dolby Pro Logic include the fact that all channels are digital, discrete and 5 of them are full-bandwidth. Additionally, the surround speakers use separate left and right channels (stereo).
Sources include all DVDs, some DBS broadcasts and some HDTV programs.
3. DTS is also a 6-channel format (“5.1-channel”). The main difference from Dolby Digital is that DTS uses less compression thus providing marginally better sound. DTS is less widespread as Dolby Digital.
Sources include some DVDs and CDs.
4. THX Surround EX – takes the Dolby Digital signal and creates another additional matrixed channel, which is then sent to one or two surround speakers, thus giving a 6.1 or 7.1 sound.
Sources: specially encoded THX Surround EX DVDs, regular Dolby Digital sources with the use of a THX Surround EX decoder.
The format you choose mostly depends on the amount of money you are willing to spend, equipment you already have and the level of performance you are willing to accept.
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