Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems USB

Interesting. And depressing. Take a look at the comments on that chip here:

The problem with all of the chips in the Burr-Brown PCM270x series is that they have high levels of jitter. A fixed-frequency master audio clock is not employed. Instead a variable-frequency master audio clock is generated based on the timing of the incoming audio data. The computer sends packets of audio data at one millisecond intervals across the USB connection. The Burr-Brown chips have internal circuitry that measures the interval between the audio packets, and then uses this information to generate a new master audio clock that matches the rate at which the computer sends packets. This system, called “adaptive” USB mode, creates several opportunities where jitter will be added to the D/A converter`s master clock:
1) Any variable frequency clock will intrinsically have more jitter than an equivalent fixed-frequency clock. In the “adaptive” USB mode, the clock in the D/A converter must “adapt” to match the rate that the computer sends out audio packets. In the case of the Burr-Brown PCM270x parts, the master audio clock comes from a VCO and inherently exhibits high levels of jitter.
2) The computer cannot send audio packets at a perfectly fixed rate. Firstly, the computer`s internal clock is not designed to have low jitter. In a market segment where costs are literally shaved to the fractions of a cent, it would add too much the cost of a computer to use a high-performance, low-jitter clock. Secondly, the inside of a computer is filled with RF interference, making it impossible for even the best of clocks to maintain their spectral purity.
3) The Burr-Brown USB-enabled D/A converter chips update (change) the frequency of the master audio clock each time a packet of audio data is received. This happens once a millisecond so there will be a strong jitter component at 1 kHz, right in the middle of the audio band. Typical measurements of the PCM270x chips exhibit jitter levels in the thousands of picoseconds, orders of magnitude worse than a well-designed one-box player.

Finally, it should be noted that D/A converter boxes based on the PCM270x series parts are limited to a maximum sample rate of 48 kHz and a maximum word length of 16 bits. So while they can transfer CD-grade audio from one`s computer, they are incapable of playing higher resolution files.

That's from Ayre's QB-9 white paper here: [url=http://www.ayre.com/pdf/Ayre_USB_DAC_White_Paper.pdf][/url]

(The last point makes it clear that 48kHz is a physical limit and will not change with firmware.)

While I am not going to buy a QB-9 (preferring to plough more money into my Meridian set-up) the design seems to have been very thoroughly thought-through and is receiving very positive reviews.

I hope Meridian catch up with this. I would rather it didn't have a USB input than had one which was not up to their usual standards of leading the field.

Nic