Electronics notes/Signal modulation

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Most broadly, pulse modulation refers to modulating or communicating or approximating a signal in a way involving pulse trains.

Note also that PAM resembles AM, PPM resembles PM, and PWM (moderately) resembles FM.

(Also, some of these parts run into analog and digital (passband) modulation, and for decent handling of said topics you're much better off on serious sites for those.)

Purposes and best uses are varied.

Side note: Pulse Code Modulation

Pulse-code modulation refers more widely to the idea of samples at at regular (uniform) intervals, and storing them as distinct values.

It that we're quantizing in time and amplitude, roughly how to store, and what it represents.

...and is not a reproduction technique, like most of this page. (reproduction was a separate thing even in the earliest development of these concepts)

Variants include:

Unqualified PCM usually means

quantization levels are linearly uniform (so ratiometric) with amplitude

...physical amplitude, so not with perceived loudness

used for sound, it means most of the bits are used on really quiet stuff (because human perception is logarithmic).

which is actually quite useful (for human consumption of audio)

also why 8-bit PCM is audibly not enough, and 16-bit is (for most things)

probably easiest raw format to deal with (particularly when storage/bandwidth are not an issue)

Directly used in Audio CDs (Red Book), in WAV files, and quite a few others.

Also used in some way or other in a number sound- or music-related electronics, although note that mu-law and a-law are seen in some older hardware

logarithmic PCM

Because of the nonlinear way we hear loudness, and given a fixed, limited bandwidth, people at different volume levels get similar signal-to-quantizing noise ratio - than linear PCM(verify)

since the original input and eventual reproduction is often LPCM (it's a choice most hardware makes), this is often used to effectively compand signal while in transmission/storage

implementations include μ-law (mu-law) and A-law

which historically implied 8-bit and often 8000Hz implementations, as used in early digital phone transmissions (see e.g. ITU-T G.711)

if storage is not a hard constraint (and harder than CPU use), then linear PCM is easier and log PCM is probably more work than it's worth

u-law and a-law are quoted as roughly comparable to the quality of 12-bit linear PCM (in what regard?(verify))

(even though technically 16-bit logarithmic still give better detail than linear 16-bit(verify), you generally don't need it)

Differential PCM

says that instead of storing the absolute values, we store the difference from the last, or to a (deterministically) predicted next value.

If the prediction is reasonable, we need to send fewer bits.

This is generally a smaller number, so allows a slight reduction in storage, but not much

ADPCM is Adaptive Differential PCM.

It takes the differential PCM output, and varies the amount of quantization of it over time.

This is a lossy format that allows you to tweak bandwidth/SNR on noisy / low bandwidth channels.

often 5-, 4-, or 3-bit

SB-ADPCM: ADPCM, but applied to distinct frequency sub-bands.

For example, G.722

splits into two bands (roughly 0Hz to 4kHz, and 4kHz to 8kHz), then applies ADPCM to each

This mainly so that you can spend bits differently, e.g. 48 of the 64 kbit/s on the lower sub-band that includes most voice energy, and 16 kbit/s on the rest. (G.722 actually has a few different target bitrates)

Even when storage/transmission is mu-law/A-law or ADPCM or some other codec, processing on endpoints is often LPCM, mostly because it's easier and faster to work with.

For context on companding: linear PCM is not clever about spreading quantization to where our (approximately logarithmic) perception hears it best. Companding basically rescales (in a reversible way) to reduce that issue.

The companding step is lossy, but since the signal to noise ratio is better than linear PCM in the same amount of bits, it provides quality improvements - and rather useful whenever you are bandwidth-limited.

Both μ-law and A-law are used in analog and digital telecommunication (in analog it was mostly for quality, in digital tranfer it was also handy for compression), in different countries.

The .au sound format uses μ-law.

See also:

• http://en.wikipedia.org/wiki/Pulse-code_modulation

• http://en.wikipedia.org/wiki/Mu-law
• http://en.wikipedia.org/wiki/A-law_algorithm
• http://en.wikipedia.org/wiki/Companding

• http://en.wikipedia.org/wiki/DPCM
• http://en.wikipedia.org/wiki/Adaptive_DPCM
• http://en.wikipedia.org/wiki/Delta_modulation

See also:

• http://en.wikipedia.org/wiki/ADPCM

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Pulse modulation

Pulse Width Modulation (PWM)

Pulse width modulation refers to switching between on and off, changing the amount it's held on, relative to the time it's held off, to vary the energy delivered (or average voltage level) over a somewhat longer term.

When implemented digitally (as it usually is), this is typically a counter plus a threshold above which to switch on.

This means there is a clock rate at which it could switch between the two.

...and a much lower speed at which the entire thing goes on and off and repeats.

For example, if it can count and switch at 1MHz, and have a 8-bit counter, then the waveform it creates repeats at (1000000/256 ~=) 3.9kHz.

This will sometimes be called the 'PWM speed', or just output speed. This isn't the best name, so check what is meant.