digital de-emphasis filter

de-emphasis filter

what does digital de-emphasis filter (tc=50/15us) mean?
And the filter corresponds to three frequencies(32khz,44.1khz and 48khz). I don't know what is the difference between three sampling freqencies. Does that mean under different sampling freqency the tranfer function is not the same? :lol:

digital de-emphasis

In old record player the emphasis technique has been developed. At low frequencies high amplitudes reduced the time of music that could be recorded. At high frequencies the signal to noise ratio was very bad. That's why those records are recorded with a "pre empased" signal. This so called RIAA emphasis filter attenuates the amplitude of low frequency signals and increases high frequency singals.

when playing this record the record players have an inverse filter, the deemphases filter correting the modifications done while recording.

In analog domane these filters are always the same.
In digital Systems the deemphases Filter is usually implemented as IIR Filter. But any way the filter characteristic for the 50/15µs filter should not change if your sample rate changes. But when changing the sample rate you must calculate new filter coefficients to achive the same filter.

You can imagine this in the following way. The filter coefficients are always normalized to the sampling frequency. So think of a low pass filter of 0.05 times the sampling frequency. At fs=1kHz the LP corner Frequency will be 0.05 * 1kHz = 50Hz. When changing the fs to 10kHz then the corner frequency with the same coefficients will be 0.05 * 10kHz = 500Hz and so on.

I am sorry but for all sample frequencies you will have to calculate new coefficients.

de emphasis

Pre-emphasis is also used on analog FM broadcasting. This is because the noise output of the detector is square law with frequency. By increasing the higher audio frequencies at the transmitter and then decreasing them at the receiver, the SNR is improved.

This same thing is done on FM microwave transmission of FDM telephone signals only the groups and supergroups are raised in amplitude as a unit at each modulation input frequency band.

digital deemphasis

Thanks !
I think the de-emphasis filter is previously realized in analog domain, and the transfer function is about (15s+1)/(50s+1).If you want to realize it in digital domain ,and the filter coefficent is different according to the sample rate!