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Transport issues: Some transports are not bit-transparent. Some DVD players have a digital volume control on the digital output. Some apply sample rate conversion to the digital outputs, and this conversion may be of a poor-quality. Also, most CD and DVD players are designed to mask or skip data when the disk cannot be read, some will attempt to re-read the data, but ultimately will skip or mask if subsequent reads fail. A dirty or scratched disk will play differently in different transports. A transport with dirty or misaligned optics will produce similar problems. Bottom line, the bits at the SPDIF or TOSLINK connector may not be identical to the bits on the recorded media. We have done some testing of transports and expect to post the results on our web site when we have more data. Some transports work very well, others do not. Unfortunately, we are finding that price is not a good indicator of transport quality.
A further word of caution when comparing digital interfaces with the Benchmark DAC1: If you connect a single transport with two different types of digital outputs to the DAC1 (SPDIF and TOSLINK for example), you WILL hear a difference between the two inputs when you flip the input selector switch on the front of the DAC1. The difference is real but the difference only persists for 500 mSec. Here is what you are hearing: When the input to the DAC1 switches, two soft-mute circuits activate to prevent pops and clicks. The mute/un-mute sequence ends in less than 100mSec (typically 60 mSec). During the mute sequence and for another 400mSec, the DAC1 UltraLock™ circuitry is disabled and the audio is pitch shifted to adjust the internal buffer lengths. The pitch shifting is completed within 500 mSec after switching inputs. The red "ERROR" LED on the front of the DAC1 is illuminated while the mute/un-mute, and pitch-shifting operations are taking place. The pitch shifting sequence is either normal-up-normal or normal-down-normal. The first half of the sequence occurs during mute, but the second half is not muted. Consequently you will hear normal audio prior to the switch, followed by a 60-100 mSec mute, followed by new input pitch shifted, followed by a 400 mS adjustment back to normal pitch. The small but abrupt change in pitch after the mute is just enough to give the impression that one input is brighter than the other. The direction of the pitch shifting sequence is determined by the relative delay between the two digital inputs. The input with the largest delay will be perceived as brighter. If the optical interface has more delay than the coaxial, it will be perceived as being brighter. After 500msec, there is no difference. Kind of interesting!
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John Siau
Director of Engineering
Benchmark Media Systems, Inc.
