Q4.3 - What is the difference between Dolby A, B, C, S, and SR? How do each of these systems work? How do they affect the sound?

The Dolby A, B, C, SR, and S noise reduction (NR) systems are non-linear level-dependent companders (compressors/expanders). They offer various amounts of noise reduction, as shown in the table below.

Dolby HF NR LF NR Number Of Active Target System Effect Effect Frequency Bands Market Year
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A 10 dB 10 dB 4 fixed Pro audio 1967
B 10 dB -- 1 sliding (HF) Domestic 1970
C 20 dB -- 1 sliding (HF) Domestic 1981
SR 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Pro audio 1986
S 24 dB 10 dB 1 sliding (HF), 1 fixed (LF) Domestic 1990
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The band-splitting system used with Dolby A NR is a relatively costly technique, although it can deal with noise at all frequencies. The single sliding band techniques used in Dolby B and C systems are less costly, making them more suitable for consumer tape recording applications where the dominant noise contribution occurs at high frequencies.

The typical on-record frequency response curves for the Dolby B NR system look something like those. The curves for Dolby C, SR, and S are similar, but the actual response levels and behavior at high frequencies are modified to extract better performance form these more advanced systems.

This attempts to show that the encoding process provides selective boost to high frequency signals (decoding is the exact reciprocal), and the curves correspond to the results achieved when no musical signal is applied. The amount of boost during the compansion depends on the signal level and its spectral content. For a tone at -40dB at 3 kHz, the boost applied to signals with frequencies above this would probably be the full 10dB allowed by the system. If the same tone were at a level of -20dB, then the boost would be less, maybe about 5dB. If the tone was at 0dB, then no boost would be supplied, as tape saturation would be increased (beyond it's normal amount).

The single band of compansion utilized with Dolby B NR reaches sufficiently low in frequency to provide useful noise reduction when no signal is present. Its width changes dynamically in response to the spectral content of music signals. As an example, when used with a solo drum note the companding system will slide up in frequency so that the low frequency content of the drum will be passed through at its full level. On replay, the playback of the bass drum is allowed to pass through without modification to its level, while the expander lowers the volume at high frequencies above those of the bass drum, thus providing a reduction in tape hiss where there is no musical signal. If a guitar is now added to the music signal, the companding band slides further up in frequency allowing the bass drum and guitar signals through without any compansion, while still producing a worthwhile noise reduction effect at frequencies above those of the guitar.

The Dolby B NR system is designed to start taking effect from 300Hz, and its action increases until it reaches a maximum of 10dB upwards of 4kHz. Dolby C improves on this by taking effect from 100Hz and providing about 15dB of NR at 400Hz, increasing to a maximum of 20dB in the critical hiss region from 2kHz to 10kHz. Dolby C also includes spectral skewing networks which introduce a roll off above 10kHz prior to the compander when in encoding mode. This helps to reduce compander errors caused by unpredictable cassette response above 10kHz, and an inverse boost is added after the expander to compensate. Although this reduces the noise reduction effect above 10kHz, the ear's sensitivity to noise in that region is diminished, and the improved encode/decode tracking provides important improvements in overall system performance. An anti-saturation shelving network, beginning at about 2kHz, also acts on the high frequencies but it only affects the high-level signals that would cause tape saturation. A complementary network is provided in the decode chain to provide overall flat response.

When the tape is played back, the inverse of the above process takes place. For an accurate decoding to occur, it is necessary that playback takes place with no offsets in levels between record and replay. IE. If a 400 Hz tone is recorded at 0dB (or -20dB), then it must play back at 0dB (or -20dB). This will help ensure correct Dolby "tracking".

Just think about it: if a -40dB tone at 8kHz was recorded with Dolby B on, then it would actually have a level of -30dB on tape. The same tone, if it were at a -20dB level, would have a level of about -15dB on tape. If the sensitivity of the tape was such that anything recorded at 0dB actually went on tape as -10dB, then you can see that the Dolby encoded tones would actually be at a lower level, and the system would have no way of determining this. It assumes 0dB in = 0dB out. Hence the signal would be decoded with the incorrect amount of de-boost.

The Dolby SR and S NR systems provide slightly more NR than Dolby C at high frequencies, 24dB vs. 20dB, but they also achieve a 10dB NR effect at low frequencies below 200Hz as well. This is obtained using a two-band approach, the low frequencies being handled by a fixed-band processor, while a sliding band processor tackles the high frequencies. This reduces the potential for problems such as "noise pumping", caused by high-level low frequency transient signals (bass notes from drums, double basses, organs), raising the sound level in a cyclic fashion. Dolby SR and S also contain the spectral skewing and anti-saturation circuits for high-level high-frequency signals that are implemented with Dolby C. The performance of the sliding band is improved over that obtained with Dolby B and C NR systems by reducing the degree of sliding that occurs in the presence of high-frequency signals. This increases the noise reduction effect available at frequencies below those occurring in the music signal.

An additional benefit of the Dolby S NR system for consumers is that the manufacturers of cassette decks who are licensed to use the system must adhere to a range of strict performance standards. These include an extended high frequency response, tighter overall response tolerances, a new standard ensuring head height accuracy, increased overload margin in the electronics, lower wow and flutter, and a head azimuth standard. These benefit users by enhancing the performance of cassette recorders as well as helping to ensure that tapes recorded on one deck will play back accurately on any other. [Witold Waldman - witold@aed.dsto.gov.au]

Improvement in signal-to-noise ratio, or any other parameter for that matter, doesn't come without a price. In the case of Dolby noise reduction, the calibration of record and playback levels is critical. Without the right setup, the wrong part of the playback transfer curve will be overlaid on the record transer curve, with the result that there's a strange bump in the overall linearity of the recording. So for any of these methods, it is essential to read and understand the Dolby setup procedure and make sure that the calibration tone (which also uniquely identifies the type of Dolby being used) is recorded at the correct level, and then the playback unit can be matched to thatlevel.

Once the levels are set correctly, the remaining sonic artifacts have to do with the tape being pushed closer to its limits in the extremes of the frequency range. The high frequency information thus sometimes seems a bit more compressed (besides being accompanied by less noise.) And, some would argue that running the audio through another dozen or more op-amps per channel must create sonic artifacts too. [David]