ES DVD Players 2005, Version 4.0 Page 19
subtle examples, noise appears as a texture or graininess not present in the
original picture. These Sony® ES Series DVD players achieve a remarkable
combination of superb fine picture detail and excellent clarity, thanks to 216 MHz
oversampling in the D/A converter.
To understand how 216 MHz oversampling can have such a powerful
effect on picture quality, it helps to understand the concepts of digital sampling
and aliasing noise.
Digital recording systems work by "sampling" the original source at a
specific rate, or "frequency." The frequency of sampling is determined by the
Nyquist Theorem, which dates back to 1928. Harry Nyquist calculated that the
sampling frequency needed to be at least twice the highest frequency in the
signal you need to record. For Compact Disc, which records audio frequencies
up to 20,000 cycles per second (20 kHz), the sampling frequency is 44,100 times
per second (44.1 kHz). Because the video signal is much more complex, the
frequencies are far higher. To capture the exceptional fine picture detail of DVD,
the black-and-white or "luminance" channel records frequencies out to 6,750,000
Hz (6.75 MHz). This means that DVD samples the video luminance channel at
13,500,000 Hz (13.5 MHz), as part of the DVD-Video format specification.
Channel Bandwidth Sampling Frequency
CD Audio 20,000 Hz 44,100 Hz
DVD-Video 6,750,000 Hz 13,500,000 Hz
Digital recording systems sample the analog input signal at a specific
rate or frequency at least twice the highest frequency of the channel.
For DVD-Video, the sampling frequency is 13,500,000 times per second
(13.5 MHz).
Nyquist sampling only works properly if the analog output is carefully
filtered of the aliasing noise that the digital process incurs. Fortunately, the
aliasing noise is consistently higher in frequency than the highest video
frequencies we want to recover. The noise appears in clusters at each multiple
of the sampling frequency, plus and minus the video bandwidth. Unfortunately,
the noise is very close to the video frequencies. The player must use a very
steep analog filter, which must be carefully constructed to pass all the video
frequencies and block all the aliasing noise.
Normally, even slight errors in the analog filter could cut the highest video
frequencies—degrading picture detail—or allow some aliasing noise to pass
through—degrading picture clarity.
The solution to this problem is to run the A/D converter at a higher
frequency than the samples on the disc. This "oversampling" design fills in the
blanks between the DVD samples with additional, calculated samples. For
example, 2X oversampling calculates and inserts one additional sample between
each original sample from the disc. 4X oversampling inserts three additional