PDA

View Full Version : Electrical characteristics of SPDIF output



cliveb
2008-02-24, 07:22
For reasons that I won't bore you with, I've been experimenting with the "Disable outputs when off" option. My hope was that when this option is selected, then when a Squeezebox is turned off the electrical signal on the COAX SPDIF output would drop to zero.

However, after some experiments, it seems that the carrier voltage is still present on the SPDIF output. What does the "Disable outputs when off" option actually do? Does it simply stop sending any SPDIF frames, while leaving the carrier voltage present?

seanadams
2008-02-24, 08:24
However, after some experiments, it seems that the carrier voltage is still present on the SPDIF output. What does the "Disable outputs when off" option actually do? Does it simply stop sending any SPDIF frames, while leaving the carrier voltage present?

Kind of. The s/pdif encoding in SB3 is sort of a hybrid software + hardware implementation. When the output is disabled there is no valid data stream, but the logic is still trying to apply the manchester encoding. This results in a waveform which is not intelligible s/pdif, but has similar electrical properties. The vast majority of receivers/dac will NOT recognize this as a valid signal, but some do.

In Transporter's V40 firmware this has been corrected (it now outputs 0 VDC when idle) and there is an open bug to do the same on SB3.

cliveb
2008-02-24, 10:11
In Transporter's V40 firmware this has been corrected (it now outputs 0 VDC when idle) and there is an open bug to do the same on SB3.
Thanks for the clarification, Sean. When the change is made to the SB3 firmware, will it also apply to the Squeezebox Receiver?

The reason I'm interested in this is because I would like to use the presence/absence of a voltage on the SPDIF output as the means to trigger a relay to switch powered speakers on/off (via a suitable circuit - I won't try to drive the relay coil directly from the SPDIF output!). The SB3 already has the PowerSwitchII plugin to achieve this, but on the SBR there is no headphone output so I'm looking for another means to determine whether the SBR is on or off.

On a slightly different but related subject: when I was experimenting with the COAX SPDIF output, it seemed to me that the output voltage is about 5V. I thought SPDIF was supposed to be 0.5V pp? (I do realise that most SPDIF receivers are happy with TTL levels, so in practice it probably doesn't matter, but nevertheless I'm curious).

seanadams
2008-02-25, 06:54
The reason you're seeing a higher voltage is because you have not terminated the signal (which is OK for just detecting the presence of a signal). Normally s/pdif would be driving a 75 ohm load. SB3 uses a 3.3v logic gate driving a resistor divider. The impedance of the load combined with this output impedance results in the correct signal level of 0.5Vpp.

For the purpose of detecting any signal (assuming a hypothetical SB3 which outputs DC when idle) you would need a circuit to differentiate between DC and AC. That could be done with a diode feeding a cap to ground, and a resistor across the cap. Use that signal to drive the base of a transistor which would pull the relay coil.

alancook
2010-11-28, 18:02
I also wish to power on a device (amp) using the digital out to switch a relay. I guessing many people do.
Do you know if anyone has a schematic and component list for a device.