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What I know is that interference increases linearly with frequency and thus noise power is concentrated at higher frequency. So, to reduce noise, pre-emphasis and de-emphasis filters are used which works by boosting the input signal before modulation and at receiver, it is deemphasized to original signal and the noise signal is not boosted and eliminated through deeemphasis filter. In this way, a great deal of noise is reduced. So, why companding is introduced then? What companding achieves more than preemphasis-demphasis?

Both emphasis and companding vary the input signal is some way or the other. Preemphasis-deemphasis vary the frequency and companding vary the amplitude range, to be precise dynamic range. But what advantage companding achieves when compared to emphasis?

In this site, it is written that most of the noise is reduced through companding but that can be also achieved through preemphasis-deemphasis. Also, if we see companding as compressing-expansion of signal, isn't the frequncy of signal gets changed? That is principle the emphasis. So, how companding is different from preemphasis-deemphasis?

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Companding is a non-linear time-domain operation that alters the signal in way that reduces the influence of perceived noise. Small amplitude signal variations are increased above the noise floor so that when these are decreased on the receiving end the noise floor is lowered.

As any non-linear operation if transmitter and receiver are not exactly matched there will be harmonics and inter-modulation distortion introduced in the signal.

Companding was commonly used to reduce 12-bits of voice information in phone systems to 8 bits for transmission, and the noise this addressed is quantization noise. It mostly works due to the non-linear aspect of our auditory system. And due to the use of complementary non-linearities, it is better applied on the digital domain.

Pre-emphasis and equalization are linear frequency-domain operations that do not introduce harmonics or intermodulation distortion into the signal. These simply change the frequency response to compensate for the frequency response of the channel. Pre-emphasis, by placing more power on the most attenuated frequencies, attempts to place the signal above the noise floor on the receiver. Being a linear system, matching is not as critical.

Although pre-emphasis and equalization are common for data communication channels, the principle is related to what was used for Dolby B/C noise reduction technology for analog audio recordings (in this case a combination of pre-emphasis and automatic gain control (AGC) just for high frequencies was used to compress the dynamic range to remain above the noise floor. An AGC is a form of companding that, although still non-linear, avoids introducing harmonic distortion).

You would not normally use companding for analog music recordings, as the wide bandwidth and required complementary non-linearities would guarantee the introduction of harmonic distortion.

However, on some analog applications in which a wide dynamic range is required, there are filtering techniques that make use of analog companding. These are called log-domain filters.

Bark space

A mixed technique that underlies all of our current lossy music formats from MP3 onward. Is the compansion of individual portions of the spectrum and the assignment of more or less bits (down to zero) to each portion of the signal spectrum. Our auditory system is highly non-linear and masks signals that lie too close to other larger signals that occur around the same time. These hearing models are used to compand the individual bands of the signal to save storage and transmission space.

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Preemphasis and deemphasis are linear operations that boost or suppress different portions of the signal spectrum. They're basically 'plain old' filters.

Companding is a nonlinear process, where a signal is boosted before transmission (or recording, I know of it from audio) when it has a low amplitude, and then undergoes a comparable reduction in strength on reception (or playback). To my knowledge it acts on the whole signal equally across the spectrum (although one could, I suppose, compand a signal by spectral component -- ugh!).

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A couple of things. First, interference does not always increase linearly with frequency. Sometimes, but not always. It depends on the characteristics of the transmission medium and the signal systems involved. Two systems that use pre/de-emphasis are LP records (RIAA) and FM broadcasting. In FM broadcasting, the system noise floor appears to be relatively constant across frequencies. However, there is much less high frequency energy in the signal being sent, so the noise appears to be a higher percentage of the received signal at the higher frequencies.

In an emphasis system, the gain for the signal changes as the frequency increases. Again using FM, for baseband frequencies above 2122 Hz the signal amplitude increases at a rate of 3 dB per octave ahead of the transmitter, and is reduced by the same amount in the receiver. So the amplitude is increased by 6 dB at 4.24 kHz, 12 dB at 8.49 kHz, etc.

The main difference between pre/de-emphasis and companding is that companding is applied equally across the entire spectrum of the source signal (as in telephone calls) or across separate parts of the spectrum (as in Dolby-A noise reduction). Within the frequency band of interest, the amount of gain boost/reduction is based on the amplitude of the entire frequency band (usually the RMS amplitude) rather than specific frequencies making up the spectrum. So if the system determines that for that instant of audio, the gain should be increased by 4.7 dB, that 4.7 dB of extra gain is applied to all frequencies, not just the highs or lows.

In short, a pre/de-emphasis system changes an individual frequency's energy based only on its frequency; a companding system changes an individual freqquency's energy based only on its amplitude.

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