SAV Joemeek

Citation :
DIGITAL NOISE

Q: Connecting my twinQ 's AES/EBU output to a MOTU 896HD causes very high level of hiss, even with input and output gain turned all the way down. Sending the outs to the MOTU as a line level signal sounds ok.

A: Many MOTU users seem to have trouble with this but it is simply a matter of setting up the MOTU correctly. Do not assume that there is a fault with either the Joemeek or the MOTU! Please refer to our separate application note about setting up a MOTU (above) - the principles explained there will mostly likely apply to other manufacturers' digital products as well.

Q: The Meek's digital output has a loud audible hiss but no audio signal is heard.

A: I assume what you mean is really bad digital noise - comparable with audio signal levels. This kind of thing is always caused by lack of synchronisation between the transmitter (ie: the Joemeek) and the receiver (Presonus, RME, MOTU or other D/A). This loss of synchronisation can happen because either:-

1. The receiver is expecting to be synchronised to something else, or
2. The receiver is expecting data in a different format (eg: ADAT instead of SPDIF), or
3. The receiver is expecting a digital signal at a higher voltage level (eg: due to the cable used), or
4. A clean data stream is not getting to the receiver (eg: due to the cable used), or
5. The receiver is not working properly, or
6. The sample rate on the Joemeek is changed without first switching the power off, or
7. The Joemeek is not transmitting a clean data stream

For cases # 1 to # 5, please study our technical note setting up a MOTU and/or contact the supplier of your receiver.

In case # 6, note that the Joemeek digital output resets itself and initialises the data stream when the power is turned on. If you plan to change the sample rate on your Joemeek, switch the power off first, change the switches and then turn the power back on. This should avoid any bursts of digital noise.

Only # 7 constitutes a fault with the Joemeek.

Q: The digital output from my oneQ works OK but the audio signal seems noisy, whatever the sampling rate and even with the input and output gains turned right down.

A: How much noise did you get and was it assessed by measurement or listening? When measuring digital noise remember you have to subtract 18dB from the reading. This is because the digital noise floor is quoted relative to Full Scale Digital (+18dBu) and not 0dBu as with analogue. So if you measure a noise floor of -75dBu, the relevant figure is -93dB FS. Remember also that your measurement must be limited to the audio band, ie: 20Hz to 20kHz. Digital noise goes way up in frequency and if you measure the full bandwidth of the noise you will get a much higher reading. A Neutrik analyser for example has a button that selects filters to limit response to audio frequencies. Without such filters you may obtain a noise measurement which is 30dB worse! Remember also that the noise figure you get will be no better than the D/A converter you use.

We measure the performance of Joemeeks using an RME D/A with a noise floor of -98dBu, together with a Neutrik Audio Analyser. Joemeek digital outputs repeatedly give a noise figure of -76dBu, which is -94dB relative to FSD. Even relative to average digital recording levels this still represents a Signal to Noise ratio of better than 80dB, which is itself typical of analogue outboard gear such as channel strips and mixers.

The digital noise is actually slightly worse at 96kHz than at 48kHz, because the electronics is working faster and the burden on the power supply is greater. However audio distortion is lower, because there are more samples per cycle of the audio waveform. That said, the difference between 96kHz than at 48kHz is only a couple of dB.
� I'm trying to connect it to an EMU 1820m using the optical connection and can't get it to work. The guys from EMU said that I have to set the card to sync to an external clock provided by the ADAT (in this case the oneQ ) but the EMU doesn't lock. Is it because the optical output is not sending an ADAT signal but an SPDIF signal instead? A: Yes, the Joemeek Optical output is SPDIF, not ADAT.

Q: Does the type of cable used affect the amount of noise?

A: Sometimes a digital cable can affect noise - it shouldn't but it does! If the cable impedance is wrong the signal reflects back down the cable and interferes with the transmitted waveform. This accentuates jitter, which can manifest itself as noise as well as distortion. It's amazing what you can get away with (providing the cable isn't too long) but you should always use decent quality cables and connectors for reliability. If you hook up the coaxial output with two bits of wet string then expect problems!

Grounding - either ground loops or inadequate ground connections - can affect performance. If you have a problem and your receiver has an optical input then use this and disconnect any other digital cables. That way grounding issues are avoided. Opt for optical. That is why it was invented!