dCS 954 User Manual

Manual for Standard Software Version 1.5x

dCS Ltd

June 2000

 

 

dCS 954 TECHNICAL INFORMATION

Anti Image Filtering

The dCS 954 offers a choice of 4 anti-image filters on most sample rates. These filters affect the ultrasonic part of the spectrum - 20 kHz upwards.

The unit is a DAC, with an output data rate set by the interface standard used. The bandwidth of the oversampling converter used is high, and so any signals that are in the input data will produce Nyquist images4 in the output signal if they are not removed by filtering. The demands on this anti-image filter can be quite severe at the lower (“normal”) sample rates - it must pass signals in the audio band (0 - 20 kHz) unimpaired, but it must remove images above Fs/2. This can result in a very sharp filter, and it is an unavoidable mathematical result that sharp filters have a poor, ringing, transient response. One effect of the ringing is to spread the energy in a transient over a significant period of time (it can be up to 1 ms). This seems to affect the stereo image that the ear would otherwise form.

One can trade off filter roll-off and energy smear - more relaxed roll-off gives less energy smear, but it may allow some of the signals in the input data to form audible images in the output signal. A signal containing Nyquist images can be corrected only by applying a sharp low-pass filter. However, as far as the ear is concerned, this may not matter. The ear can tell the frequency of a signal - up to a point. As the frequency rises, the accuracy with which the ear can tell what the frequency is decreases, and above a limit, all the ear can tell is that there is a signal, and it is above ... kHz. It can tell no more. So - it may be that a small amount of Nyquist imaging is acceptable to the ear.

The filters that we have included give increasingly good energy smear performance, and consequently have increasingly relaxed roll off. FiLT1 gives the sharpest roll off, with no Nyquist images, but the worst energy smear. Then as the number increases the smear decreases, but the imaging increases. Try them, to see which you prefer.

You may find that for different material, different filters are appropriate - and you may find that for different stages in the recording and mastering process, different filters are appropriate.

Our users tell us that they find the ability to select different anti-image filters very useful. Generally for classical music, number 2 is preferred while 3 & 4 are popular with users recording rock and jazz. Opinions differ widely – so try them for yourself.

The dCS 954 uses linear phase FIR filters to avoid the limit cycle problems that come with many IIR filters. Linear phase gives filters a symmetrical transient response before and after a transient (“pre-ringing”). The passband may or may not have a ripple5, depending on the filter being used. The stop band is typically below –110dB0 and can be as low as –130dB0.

4

5

See, for example “Principles of Digital Audio”, 3rd Edition, by Ken C Pohlmann (McGraw-Hill Inc, 1995) Filters always have some ripple. For “zero ripple” filters this is in the ∝dB to pdB region.

Manual part no: DOC136954 iss 2B1

Page 38

file 135954ma2b1.pdf available from website

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