CD player components: Processors, Buffers …

Walid El jebbari

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

Oversampling using a DSP• i

Oversampling using a DSP• i

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)

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

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 laser’s 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

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

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/