Multimedia Systems

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Multimedia Systems Interfaces

• Slides adapted from material created by Chris
  Wren and Paris Smaragdis

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Images

• Transforms (cont.)
• Sampling (cont.)
• Filter examples

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Affine Transforms
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Sampling
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Downsampling

• From high sample rate to low
• Downsampling by 2
• If we pick one out of two samples we will
  introduce aliasing
• In order to get around it we can remove the high
  frequencies first and then pick samples
• For downsampling by M
• Lowpass to 1/M
• Then pick one out of M samples
• Only works for integer M
• Loss of information (high freqs)
• We can double up as well
• To downsample by M 8 we can downsample three
  times by M 2
• Use MATLABs decimate for this

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Upsampling

• From low sample rate to high
• For upsampling by L
• Put L-1 zeros between each sample
• Lowpass filter to 1/L
• Only works for integer L
• Without filtering we get these noisy clicks
• No loss of information
• We can combine multiple upsamplers
• To upsample by L8 we can upsample three times by
  L2
• Use MATLABs interp

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What if the ratio is not integer?

• Describe it using two integers
• E.g. to go from 44.1kHz to 48kHz
• Upsample by 1.0884
• This is 480/441
• We can upsample by 480 and downsample by 441!
• Efficiency
• There are ways to deal with not having to
  upsample by 480
• Using tricky bookkeeping we can consolidate the
  two operations into one
• For huge inputs it can be brutal otherwise!
• Thats why CDs are 44.1kHz and DATs are 48kHz, to
  make resampling a pain
• For nasty ratios we might get to lose a little
  bit of information
• But it all sounds good pretty much always
• Use MATLABs resample

44.1kHz
21168 kHz
Upsample by 480
21168 kHz
Downsample by 441
48 kHz
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As we recall from last class, we should consider
the Nyquist rate when sampling. A very good
example can be found in wikipedia under Nyquist.