CD player components: Processors, Buffers

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CD player components Processors, Buffers

• Walid El jebbari

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Oversampling using a DSP

• Oversampling means using a sampling frequency
  greater than that dictated by the Nyquist theorem
  
• It results in great performance gains an
  increase in signal-to-noise ratio and a decrease
  in the quantization noise
• Done using a digital signal processor (digital
  filter) which takes in audio samples, performs an
  operation on them, and then outputs audio
  samples.
• Oversampling can be viewed as interleaving zeros
  between each sample with additional samples

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Oversampling using a DSP

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Oversampling using a DSP

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Output Filtering

• The role of the output filters is to smooth out
  the waveform from the digital-to-analog
  converters
• the output filter stage has witnessed a great
  deal of improvement since 1982
• Early 16-bit players used brickwall
  reconstruction filters
• These filters had a very sharp cutoff
  characteristic and held the signal gain close to
  unity
• The problem was that they had big phase
  nonlinearities at high frequencies, and
  high-frequency group delay (change in phase shift
  with respect to frequency)

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Output Filtering

• current players have taken advantage of digital
  oversampling filters placed upstream of the DAC
  along with a gentle analog reconstruction filter
• These filters have a gentler cutoff
  characteristic than the brickwall filters,
  because the oversampling filter shifts noise
  outside the audio band
• Its design is noncritical and low-order--which
  guarantees excellent phase linearity
• phase distortion can be held at 0.5 over the
  entire audio band

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Buffer memory

• CD players incorporate a buffer memory that
  reads ahead on the disc stores up to 45 seconds
  of audio as a CD plays
• When the player is bumped or jostled, it
  interrupts the lasers ability to read the data
  from a CD
• The buffer memory temporarily reads the contents
  of the buffer so playback can continue
  uninterrupted while the laser assembly regains
  its bearings

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The ability to read different formats

• CD-R was designed to be read by an infrared 780nm
  laser
• DVD uses a visible red 635nm or 650nm laser,
  which aren't reflected sufficiently by the
  organic dye polymers used in CD-R media
• As a result, many DVD players can't read CD-R
  media
• Some DVD players come with two lasers so that
  they can read CD-R one infrared for CDs, one
  visible for DVDs

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References

• www.tc.umn.edu/erick205/Papers/paper.html
• www.answerbag.com/c_view.php/2012
• www.bestbuy.ca/learnmore/AskTheExpert/en/portable_
  cd_players_faqs
• www.cdrfaq.org/