6
$\begingroup$

Hi I'm taking a multimedia systems course and I'm preparing for my exam on tuesday. I'm trying to get my head around LPC compression on a general level, but I'm having trouble with what is going on with the linear predictive filter part. This is my understanding so far:

LPC works by digitising the analog signal and splitting it into segments. For each of the segments we determine the key features of the signal and try to encode these as accurately as possible. The key features are the pitch of the signal (i.e. the basic formant frequency), the loudness of the signal and whether the sound is voiced or unvoiced. Parameters called vocal tract excitation parameters are also determined which are used in the vocal tract model to better model the state of the vocal tract which generated the sound. This data is passed over the network and decoded at the receiver. The pitch of the signal is used as input to either a voiced or unvoiced synthesiser, and the loudness data is used to boost the amplitude of this resulting signal. Finally the vocal cord model filters this sound by applying the LPC coefficients which were sent over the network.

In my notes it says that the vocal tract model uses a linear predictive filter and that the nth sample is a linear combination of the previous p samples plus an error term, which comes from the synthesiser.

  • does this mean that we keep a running average of last p samples at both the encoder and decoder? So that at the encoder we only transmit data that corresponds to the difference between this average and actual signal?

  • Why is it an a linear combination of these previous samples? My understanding is that we extract the loudness, frequency and voiced/unvoiced nature of the sound and then generate these vocal tract excitation parameters by choosing them so that the difference between the actual signal and the predicted signal is as small as possible. Surely an AVERAGE of these previous samples would be a better indication of the next sample?

If there are any holes in my understanding if you could point them out that would be great! Thanks in advance!

$\endgroup$
13
$\begingroup$

LPC voice coders (starting with the old LPC10 standard, which seems to be the one you refer to here) are based on the source-filter model of speech production. Speech can be characterized by the following properties:

  • The raw sound emitted by the larynx (through vibration of the vocal folds, or just air flowing through it, the vocal folds being opened).
  • The transfer function of the filter achieved by the articulatory system, further filtering this raw sound.

Early LPC coders (LPC10) adopt the following model of those two steps:

  • The larynx emits either white noise, characterized by an amplitude $\sigma$ ; or a periodic impulsion train, characterized by an amplitude $\sigma$ and a frequency $f_0$
  • The transfer function of the articulatory system is of the form $\frac{1}{1 - \sum_k a_k z^{-k}}$, and is thus entirely characterized by the coefficients $a_k$

The principle of early LPC coders (such as LPC10) is thus to estimate these parameters from chunks of the incoming audio signal ; transmit these over the network ; and have a sound generator reproduce a sound from these parameters on the receiver. Observe that in this process, nothing of the original audio samples is actually transmitted. To make a musical analogy, it's like listening to a piano performance, transcribing it, sending over the score, and getting someone to play it on the other end... The result on the other end will be close to the original performance, but only a representation has been sent over.

does this mean that we keep a running average of last p samples at both the encoder and decoder?

No, this is not how it works. On the encoder side, we run a process known as AR (autoregressive) estimation, to estimate the set of coefficients of the AR filter that matches best the spectral envelope of the input signal. This is an attempt at recovering the coefficients of the filtering that has been performed by the articulatory system.

These coefficients are sent over the network (along with pitch, the voice/unvoiced flag, and loudness). The decoder uses those coefficients to filter a synthetic excitation signal, which can be either white noise (unvoiced frame) or a comb of periodic impulsions (voiced frame). These coefficients are used in the following way on the decoder to recover the output signal $y(n)$:

$$y(n) = excitation(n) - \sum_k a_k y(n - k)$$

Observe that since this is an all-pole, IIR filter, the samples linearly combined are previous samples produced by the filter.

So that at the encoder we only transmit data that corresponds to the difference between this average and actual signal?

There is no "averaging" and there is no transmission of a difference signal. The decoder is a sound synthesizer with some parameters ; and the encoder searches for the set of parameters for this synthesizer that most closely matches the input signal.

Why is it an a linear combination of these previous samples?

Other options would be possible indeed here, but the advantage of using an auto-regressive model are the following:

  • In this model we use an all-pole IIR filter to simulate the articulatory system. This is a good model, since it can capture strong spectral dips and peaks with a small number of coefficients ; and the articulatory system is indeed capable of emphasizing / attenuating narrow bands of frequencies (See formants). If we had used an all-zeros FIR filter, we would have needed many more coefficients to accurately capture the kind of filter responses that the articulatory system can achieve.
  • On the encoder end, the problem of of estimating the filter coefficients is computationally efficient - it can be done using the Levinson-Durbin recursion on the first few samples of the autocorrelation. More complex linear models (ARMA) or non-linear models are more computationally expensive or intractable.
  • On the decoder end, the synthesis is very simple indeed - we just need enough memory to keep track of the n previous samples emitted by the decoder.

Surely an AVERAGE of these previous samples would be a better indication of the next sample?

This is not true. For example, assume the input signal is a sine wave. The prediction $\hat{y}(n) = (2-\omega^2) y(n - 1) - y(n - 2)$ has zero error, while $\hat{y}(n) = \frac{y(n - 1) + y(n - 2)}{2}$ is wrong most of the time (in particular, it does not capture the fact that the sine wave is strictly increasing half of the time, strictly decreasing the rest of the time, so $y(n)$ shouldn't be "between" the previous values). Speech signals are not sinewaves of course - but you can squeeze much more modelling power out of a model with $p$ parameters (order p AR), than out of a model with zero (averaging).

Also, it's worth keeping in mind that there's a mathematical definition of "better" (minimizing the expected value of the squared of the error), which yields a procedure for finding the optimal value of the coefficients.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.