The discontinued Logitech Squeezebox Touch is a fine product for multi-room audio streaming, though sadly discontinued. The Touch is limited to a maximum audio resolution of 24-bit and 96Khz – or is it? While this is true of the internal DAC (Digital to Analogue Converter), an audio enthusiast known as Triode set about modifying the firmware to output higher resolutions.
Another aspect of his work was to enable use of the USB port on the Touch, which was originally designed for attaching storage, to support an asynchronous USB DAC. The idea of async USB audio is that the clock which controls the decoding is in the DAC and independent of the source. There is an explanation by Vincent Kars here:
In this mode an external clock is used to clock the data out of the buffer and a feedback stream is setup to tell the host how much data to send.
A control circuit monitors the status of the buffer and tells the host to increase the amount of data if the buffer is getting too empty or to decrease if it’s getting too full.
Since the readout clock is not dependent on anything going on with the bus, it can be fed directly from a low jitter oscillator, no PLL need apply.
Is async USB audio really necessary? Is any resolution above 16/44 necessary for playback? I am sceptical, but after getting hold of a Teac UD-H01 which supports both async USB and resolutions up to 24/192 I thought it would be fun to try it.
I followed the steps here which worked perfectly. They involve installing a Squeezebox app called Enhanced Digital Output, and installing a modified Linux kernel (the Touch runs Linux). The modifications are reversible, which is reassuring.
I then tried one of the the few Flac-encoded music files I have in 24/192 format. I still cannot tell you whether either the format or the asynchronous aspect makes a difference; but I can say that the sound is exceptionally good, though it also sounds excellent with well-mastered 16/44 sources.
Subjectively it improves on the internal DAC in the Touch, with deeper bass, a more spacious sound, better separation between instruments, more natural vocals, and all the usual hi-fi clichés.
Leaving that aside though, kudos to Triode for his achievement.