dfpwm
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— | dfpwm [2015/06/25 15:36] (current) โ created - external edit 127.0.0.1 | ||
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+ | DFPWM (Dynamic Filter Pulse Width Modulation) is an audio codec created by Ben " | ||
+ | |||
+ | DFPWM is recognisable, | ||
+ | |||
+ | It depends on the implementation as to what all the parameters are. Testing has shown that 8 bits per sample works better than 16 bits. The codec is not frequency-dependent, | ||
+ | |||
+ | If necessary, you can get away with converting your streams to raw 1-bit, or you could possibly assume a static-strength low-pass filter. | ||
+ | |||
+ | The implementation in Computronics handles streams using Ri=7, Rd=20, 8 bits per sample, and LSB stored first. | ||
+ | |||
+ | ==== Specification ==== | ||
+ | |||
+ | DFPWM, at its simplest, works like this: | ||
+ | Decoding: | ||
+ | smp(t) <- Predictor(bit(t)) | ||
+ | Encoding: | ||
+ | if smp(t) > LastPredictor OR (smp(t) = LastPredictor = 127): | ||
+ | bit(t) <- Predictor(1) | ||
+ | else: | ||
+ | bit(t) <- Predictor(0) | ||
+ | |||
+ | where bit is either LOW or HIGH, and smp โ [LOW, HIGH]. (Apologies for using the โ, it's just that if I try to do "less than or equal", | ||
+ | |||
+ | === State === | ||
+ | |||
+ | Firstly we need to give some types: | ||
+ | * Let q,s be signed integers. | ||
+ | * Let b' be a single bit, either 0 or 1. | ||
+ | * Let Ri,Rd be chosen constant signed integers. ((7,20) is reasonable for 8 bits per sample.) | ||
+ | |||
+ | Then we need to assign meanings: | ||
+ | * Let q be the " | ||
+ | * Let s be the " | ||
+ | * Let Ri be the strength increase. | ||
+ | * Let Rd be the strength decrease. | ||
+ | |||
+ | Ri and Rd are constant (and, until someone discovers a better set of values, will always be 7 and 20 respectively). q and s vary. | ||
+ | |||
+ | From here we can define the predictor. | ||
+ | |||
+ | === Predictor === | ||
+ | |||
+ | To simplify this, we will define this in terms of signed 8-bit samples. | ||
+ | |||
+ | == Input comprehension == | ||
+ | Let b' be the previous instance of b, initialised to 0 to simplify implementation. | ||
+ | |||
+ | Let t be the " | ||
+ | |||
+ | == Charge adjustment == | ||
+ | Let q' be an integer such that | ||
+ | > q' <- q + (s*(t - q) + 128)/256 | ||
+ | |||
+ | If q == q', and q != t, then: | ||
+ | > If t < q: q' <- q' - 1 | ||
+ | > If t > q: q' <- q' + 1 | ||
+ | |||
+ | This is done to ensure that the " | ||
+ | |||
+ | Then set q <- q'. | ||
+ | |||
+ | == Strength adjustment == | ||
+ | Let r,z be integers such that: | ||
+ | > If b == b', then r = Ri, z = 255 | ||
+ | > If b != b', then r = Rd, z = 0 | ||
+ | |||
+ | Let s' be an integer such that | ||
+ | > s' <- s + (r*(z - s) + 128)/256 | ||
+ | |||
+ | If s == s', and s != z, then: | ||
+ | > If z < s: s' <- s' - 1 | ||
+ | > If z > s: s' <- s' + 1 | ||
+ | |||
+ | Then set s <- s'. | ||
+ | |||
+ | === Filtering === | ||
+ | |||
+ | You can do anything here, within reason. These methods are by no means the best way to deal with the noise you get. | ||
+ | |||
+ | These notes are based on the C implementation. | ||
+ | |||
+ | == Antijerk == | ||
+ | |||
+ | If the current target and the previous target are different, output the average of the current and previous results from the predictor; otherwise, output the value directly. | ||
+ | |||
+ | == Low-pass filter == | ||
+ | |||
+ | outQ <- outQ + (expectedOutput - outQ) * 100/256, essentially. | ||
dfpwm.txt ยท Last modified: 2015/06/25 15:36 by 127.0.0.1