dCS 974 User Manual

Manual for Software Version 1.0x

dCS Ltd

May 2001

 

 

PCM Input and/or Output Performance

Filtering

The normal filtering considerations of passband ripple, cutoff frequency and rate, out of band (stop band) suppression, and transient/phase response apply to sample rate converters.

The dCS 974 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” and “post ringing”). The passband may or may not have a ripple14, depending on the conversion and/or filter being used. Cutoff frequency is >40% of the lowest sampling rate used in the conversion (input or output).15 The stop band is typically below –110 dB, but varies with conversion, and can be as low as –130 dB. The frequency responses of two commonly used conversions (96kS/s ⇒ 44.1kS/s and 48kS/s

⇒44.1kS/s) are shown in Figure 41 and Figure 40 on page 99.

The transient responses of the filters in the 96 kS/s ⇒ 44.1 kS/s conversion are shown below.

Figure 19 – Transient Performance of 44.1 kS/s Filter Options

Use of higher sample rates gives much tighter transient response, with much less energy smeared into the pre and post ringing. The transient responses of the 96 kS/s output filters are shown below:

14Filters always have some ripple. For “zero ripple” filters this is in the ∝dB to pdB region.

15For conversions where the lowest rate used is 16 kS/s, 22.05 kS/s or 24 kS/s some conversions cut off at 33% of the lowest sample rate.

Manual part no: DOC1241121A1

Page 73

Document No: OS-MA-A0124-112.1A1

Contact dCS on + 44 1799 531 999

email to: more@dcsltd.co.uk

(inside the UK replace + 44 with 0)

web site: www.dcsltd.co.uk