The problem can come up often in Computing, that instead of having a fixed-length encoding for numbers, we may want to encode a majority of integers which lie in a small range, but out of which a progressively smaller number have larger values. This can lead to variable-length encoding schemes, and MP3 sound compression, logically, is one place where this happens.
If one is trying to guess at what encoding was used, the fact that can stymie a person is, that many methods exist to accomplish exactly that. Huffman Encoding has as a problem, that although the higher-value integers are assigned longer bit-sequences, the relative frequency with which these higher integers will occur, is not the inverse, of their bit-length. This can also be why, a non-default arrangement can be made, that if the size of the integer reaches 15, a full-length value needs to follow.
I have now finally learned, that with MP3 compression, at least the integers smaller than 15, Huffman-Encoded, are intended to be the default case, and values at or above 15 are intended to be the exception. Thus, if the scaling factor is increased, for sure the bit-length of the stream will decrease, until the bit-rate is achieved, that the user set. I got that.
But Why, then, You May Ask, does Dirk choose a different way to accomplish the same thing, and so often?
My answer would be, that formal solutions are often good at compressing the size of integers when those lie in a certain range, but if one value appears in the stream which is much larger than the average, those variable-length encoding schemes can become monsters. ( :1 )
As an example, I read that ‘FLAC’ will record the Linear Predictive Encoding coefficients for a frame accurately, and that this scheme will then Rice Encode the residual each time.
Pure Rice Encoding means, that a remainder of fixed bit-length is encoded with each sample, but that it is given a (variable-length) prefix encoded in straight unary, which states what the multiple of the tunable parameter is (the quotient), that fits into the fixed-length remainder. This choice of a pure unary prefix is questionable, for the reason that I just stated above.
Now, I know that there is also Exponential-Golomb encoding, which like Huffman Coding has a bit-length that grows with the size of the integer to be encoded. But Exponential-Golomb generally produces a bit-length twice as long, as what it would take just to write out the integer on paper.
And so at least a slightly more sophisticated form of encoding exists, which is called Golomb-Rice Encoding, which is essentially Rice Encoding, but in which the prefix, which states the quotient, is prepended in Exponential-Golomb format. Why would they not use it?
And, since it is possible just to put a prefix before an integer in unary, that states its length, an approach which I would be tempted to use, would be just to assume that this unary prefix should be multiplied by a factor such as 3, to arrive at the true length of the integer.
But then a problem with that would be, the fact that this type of prefix would need to be at least 2 bits long, for non-zero values, followed by this multiple of bits belonging to the value, as a minimum. So it will not compress very small values well.
And the reason for this would be the fact, that it would no longer be certain then, that the first bit which actually belongs to the integer, will always be a (1), the way it is with Exponential-Golomb.
And, while I tend to view such encoding schemes as arbitrary, the fashion these days is, always to select a formally-defined one.
In general, my approaches will work well, if a substantial number of values are high.
1: ) And what ‘FLAC’ will do in such a case, is just switch the type of the frame this happens in, to a type ‘VERBATIM’ frame. In other words, FLAC would just decide ‘This is one frame we cannot compress’.
FLAC also has a mode, in which each sample is stated as a delta, from the previous one. This corresponds to an ‘LPE’ with one predictor, the coefficient of which is just equal to (+1), relative to which the current value is just the residual…
(Edit 05/25/2016 : ) This is another posting of mine, in which I explain an additional detail about MP3 compression.
Further, ‘FLAC’ is able to encode some of its frames as using LPE, with a variable number of coefficients. I.e., When set to compress more, it will spend its CPU time trying encodings with (!) 6 or more predictors, and will store those in cases where doing so led to more-compact encoding.
While a set of 4 or more coefficients needs to be computed specifically for one frame, via a Statistical Regression Analysis, I have read that for 1, 2, or 3, FLAC just uses a standard set of them. For 1, that will be . For 2, that will be . It might seem like a purely academic exercise, to know a standard set of coefficients, which will generally work well if there are only 3 of them. But in fact, having this available offers a non-trivial advantage, over having to store those in the compressed stream.
Since we would presumably be multiplying signed, 16-bit samples with signed, 16-bit coefficients, it will be helpful if the latter only need to fall into the fractional range of ( -1.0 … +1.0 ) . The reason for this is the fact that If we needed to store coefficients which are allowed to exceed ( + 1.0 ) , Then we are blowing another bit of precision just so that one coefficient could do so.
The most recent coefficient will still exceed ( +1.0 ) when there are 3. But as soon as there are 4, none of them would exceed ( +1.0 ) anymore. Therefore, all the coefficients which must be stored, when their number reaches 4 or more, can be made more precise, just because there is a standard set for when we have 3.