Basically, digital filters are used in CD playback to eliminate "images" of the audioband spectral content at multiples of the sample frequency. Sony posits that changing the cutoff characteristics of the digital filter changes the tone of the music. The filters can roughly be classified as sharp- (fast-) and slow-rolloff types. Sharp rolloff whacks everything above 22.05kHz—Ye Olde Bricke Wall. Slow-rolloff filters minimize the time-domain errors endemic to sharp filtering and so, it is said, improve the sound. Sony reminds us that although the filter primarily changes characteristics above the audible frequency range, the effects of those characteristics can still be heard. In comparison to "conventional" digital filters, Sony's VC24 handles double the number of steps with 24-bit word-length precision and direct 8x oversampling at a 16-bit word length.
The SCD-1's STD (Standard) filter sharply attenuates the signal above half the sampling rate (22.05kHz). Filter 1 ("dfil-1") "smoothes interpolation computations during 8x oversampling." Previous generations of chips, we are told, managed 8x oversampling by doing 2x oversampling three times. With VC24's "superior computing capabilities" (a little self-congratulation never hurt anyone), "direct 8x oversampling is performed simultaneously. The result is smoother processing and therefore clearer sound quality."
Filter 2 has the job of performing "more sophisticated" computations than standard digital filters. As much as possible, "it takes the longest number for calculation processes." This is interesting. Conventional filters truncate or round up and down before passing data to the next stage; "dfil-2" adjusts the length of the input number to match the noise shaper before passing it along. "This way, all nonlinear portions in the computation process are eliminated. Thus, no quantization noise is generated and the data read from the CD retains all of its integrity. Also, there are no longer any nonlinear computations between stages, enabling a powerful sound." These guys were obviously racking up the overtime.
Filter 3 is a new implementation Sony calls an even-number digital filter to take best advantage of the direct 8x oversampling with a 224-order filter. Their explanation is cogent: "Even-number filters use a completely different computation process from conventional odd-number filters. When performing 8x oversampling, an odd-number filter does not process the original data read from the CD. Instead, it makes computations using the interpolation data, which is a sample of 7. In the case of this even-number filter, computation processes are made using the original data read from the CD, and similarly using the interpolated data from the 7 samples. Again, thanks to the VC24's superior computing capability, the 224-order filter enables flat amplification characteristics from 0Hz to 17kHz, regardless of whether [it is] a slow-rolloff type filter or not." That also means they can chop off the top end higher at something over 26kHz.
Filter 4 is a "performance-enhanced" version of "dfil-2." The computational word length is increased by 4 bits, and the filter degree is increased twofold. It's also based on an even-number filter. "These specifications allow dfil-4 to achieve a flat frequency characteristic up to 18kHz for fresh, energetic sound that is full of complexities."
Here's how Sony describes the filter actions in the draft copy of the manual included with the SCD-1:
"STD (Standard) provides a wide frequency range and spatial feeling, as it holds the most information among the five filters and is suitable for classical music.
• Filter 1: Clear (slow rolloff) provides smooth and powerful sound with clear image position and is suitable for jazz band and jazz vocals.
• Filter 2: Plain (slow rolloff) provides fresh and energetic sound with rich vocal expression suitable for vocal-enhanced music.
• Filter 3: Fine (slow rolloff) provides a well-balanced natural sound having a feeling of large scale and rich reverberation suitable for listening to any music when you are relaxed." (That's the one!) "
• Filter 4: Silky (slow rolloff) provides a feeling of wide scale with associated subtleties suitable for classical music, especially if you enjoy fine string touches."
Right on target Sony!
