Is Your Transport Giving You All The Bits?

As a manufacturer of external digital-to-analog converters, Benchmark initiated a survey of DVD transports to determine which to recommend to their customers. The research was focused on bit-transparency – the ability to deliver the audio data on the disc to the digital output, bit-for-bit. Our research was performed with an Audio Precision SYS-2722 test station – a state-of-the-art analyzer for testing audio electronics.

Surprisingly, many DVD transports currently on the market fail to transmit the exact digital audio data bit-for-bit as it appears on the DVD. Of the DVD transports Benchmark has tested (8 as of this writing), all of them modified high-resolution audio data before sending it to the digital output (although they did pass 'Redbook', CD-quality 44-kHz, 16-bit data without modification). Let's examine what all of this means to your playback system.

Cause and Effect

Transports modify the high-resolution data in order to comply with digital copyright obligations to the publishers and distributors of music and video discs. The music and film industries are trying to prevent bit-for-bit copying of the recordings. Consequently, manufacturers of DVD transports have been forced to reduce the resolution of digital outputs to prevent digital duplication.

However, the sonic consequences of these modifications can be devastating to audio quality. We are able to measure sample rate, bit depth, and conversion quality with our Audio Precision test equipment. In almost all cases, the data modification includes poor quality sample-rate conversion (SRC) and word-length truncation. These processes defeat the benefits of high-sample-rate and/or high-bit-depth source material, and severely degrade sonic quality. Although the SRC and word-length reduction is being applied to reduce the quality of the digital output to CD resolution, it actually degrades the sound quality far beyond that of properly mastered CD resolution. In fact, these transports will deliver higher performance from a CD than from a high-resolution DVD.

Sample-Rate Conversion

Sample-rate conversion (SRC) is a process which changes the sample rate of the digital audio data. The sonic consequences of this process can range from 'benign' to 'detrimental'. Modern high-precision digital signal processing can achieve transparent SRC with virtually no sonic modification. However, poorly executed SRC algorithms can cause severe distortion.

High-quality SRC (sample-rate conversion) can increase the quality of performance of a digital system. SRC can improve filter performance and can provide exceptional jitter attenuation. For example, Benchmark's UltraLock system isolates the jitter of the incoming clock and optimizes filter performance by using asynchronous sample-rate conversion.

Unfortunately, the SRC processing in the transports tested was of poor quality. The distortion that results can be devastating to the quality of the playback system. Also, it is often re-sampled to 44.1 kHz, which inherently eliminates all the advantages of playing high-sample-rate audio discs.

Bit-Depth Reduction

The bit-depth of a digital audio recording determines the amount of noise that must be intentionally applied to accurately record and reproduce the audio. The larger the bit-depth, the lower the amplitude of noise required. Therefore, increasing the bit-depth of the recording and playback system will increase the maximum 'dynamic range' that the system can achieve.

'Dynamic range' is the difference between the amplitude of the noise floor and maximum output of an audio device. It is similar to 'signal-to-noise ratio', differing only by measurement technique ('dynamic range' is less forgiving).

For a digital recording to accurately record and reproduce analog audio signals, it must be 'dithered'. 'Dithering' is a process where noise is added to the audio when converting to or from analog or applying DSP. This noise eliminates quantization distortion – inaccuracies that result from representing non-discrete values as discrete values. Quantization distortion is a very unflattering type of digital distortion that can plague a digital recording. Dither effectively eliminates quantization distortion and gives digital audio the ability to accurately reproduce very low-level signals that are well below the noise floor (just like an analog system).

Most DVD transports reduce 24-bit digital recordings to 16 bits by truncating the 8 'least-significant-bits' (LSB's). In other words, these 8-bits are simply removed without dithering. This type of truncation adds quantization errors that result in significant levels of distortion.

What To Do?

DVD transports fail to deliver high-resolution digital audio to an external DAC. In most cases, they only deliver bit-transparent (unmodified) digital audio from standard CD's. So, what options are available for high-resolution digital audio playback?

There may be an alternative solution for high-resolution DVD-A and DVD-V playback: a computer. Currently, there are several media players that will play 24-bit audio files at sample rates up to and above 96 kHz, such as Foobar2000, iTunes, and Media Monkey. Several websites now offer high-resolution downloads for computer playback.

Benchmark is currently searching for software that will playback high-resolution DVD-A and DVD-V discs bit-transparently. We will report any progress in upcoming feedback articles. We also appreciate any information you can offer us regarding these (or any) issues. Keep in touch!