Electronics notes/Signal modulation
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.
- 1 Side note: Pulse Code Modulation
- 2 Continuous wave modulation
- 3 Pulse modulation
- 4 See also
- 5 Practical notes
- 5.1 PWM Types
- 5.2 PWM for average energy
- 5.3 Hardware PWM, software PWM, and inbetween
- 5.4 PWM or PDM as a simple DAC
- 5.5 On PWM distortion
- 5.6 On PWM filtering
- 5.7 PWM IC notes
- 5.8 Comparing PDM and PWM
- 5.9 Combining PWM or PDM outputs for bit depth
Side note: Pulse Code Modulation
Pulse-code modulation refers to using samples at at regular (uniform) intervals, and storing them as distinct values.
It's only really about quantized signal storage - about how to store it, and what it represents, and uses like transmission (e.g. multiplexing signals on a telephone line).
...and is not a reproduction technique, like most of this page. (reproduction was a separate thing even in the earliest development of these concepts)
Unqualified PCM usually means LCPM (Linear PCM)
- quantization levels are linearly uniform, i.e. ratiometric with the represented signal
- this is the easiest raw format to deal with, particularly when you have enough storage/bandwidth
- used for sound, it means most of the bits are used on really quiet stuff (because human perception is logarithmic). This is part of why 8-bit PCM is audibly not enough, and 16-bit is for most things.
- 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 also common.
μ-law (mu-law) and A-law are two specific types of logarithmic PCM
- since input and output is often linear, this is often used to effectively compand signal while in transmission/storage
- Because of the nonlinear way we hear loudness, and given a fixed, limited bandwidth, we get better signal-to-quantization-noise ratio than linear PCM
- mu-law and a-law often imply 8-bit implementations (see also ITU-T G.711), as e.g. used in early digital phone transmissions, which in quality are roughly comparable to the quality of 12-bit linear PCM
but if storage is not a hard constraint (and harder than CPU use), linear PCM is easier and log PCM is probably more work than it's worth
- (even though technically 16-bit logarithmic still give better detail than linear 16-bit(verify), you generally don't need it)
Delta Modulation / Delta PCM
- says that instead of storing the absolute values, we store the difference from the last.
- This is generally a smaller number, so allows a slight reduction in storage, but not much.
ADPCM is Adaptive Delta PCM.
It takes the delta PCM output, and varies the amount of quantization of it. This is a lossy format that allows you to tweak bandwidth/SNR on noisy 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.