Post by solderdude on Sept 22, 2013 9:26:24 GMT
Some background info on SPDIF signals and handling.
SPDIF is a serial signal that is always alternating. The average value (voltage level) of the electrical signal is always around 0V where the peak voltage levels can vary within certain limits (+/- xVolt). When the voltage amplitude(level) is too small errors will occur or the receiver won't operate or show drop-outs/ticks.
When the level is too high (usually due to improper termination = load impedance) an amplifier may also experience problems.
Within a certain voltage range all would be fine (level wise) as the 'decision point' is based on the middle of that voltage and needs a minimal voltage to be detected.
The fact that the average level of the signal is always (very close to) 0V over a minimal amount of time is very handy for engineers as the 'decision point' above and below one considers a certain voltage 'high' or 'low' is easily determined.
Also It is very handy to know that the 'high' and 'low' voltage levels do NOT represent the actual audio bits that are used by the DAC chip but are just a carrier. The bits are 'decoded' and the timing is used (in different ways for different DAC's) to synchronise with the transmitter 'frequency'.
When reflections, that are ALWAYS present but differ in amplitude (severity) depending on the termination on BOTH ends (transmitter and receiver side) as well as the properties of the used cable, when longer than 1 or 2 meters, distort that decision point jitter may be the result.
How much this influence is depends on sample (data) frequency, cable length and termination as well as output level of the 'transmitter' as the input range of the 'receiver' side.
For this reason it is VERY hard to predict HOW a certain combo 'works' and depending on the used DAC that jitter may or may NOT be affected by that jitter. That all depends on the immunity to jitter of the used DAC. This is determined by HOW the clock of the actual DAC CHIP used is (re)created.
When improper termination is present (hard to 'see' without specialised or home made test equipment) and the levels are within the level 'range' of the DAC all should be well.
In certain cases all that is needed is proper termination (but there is no way to really find out in which case without knowledge or test gear) is all that is needed.
the attenuation circuit may help reduce jitter IF the output voltage is high (of the source) and the receiver is sensitive enough to accept lower input levels due to A: better matching, B: jitter reduction by attenuation.
It also may help if the source level is too high and termination on the receiver end is wonky/bad and or the input of the receiver is 'over shouted' by the input level and reacts by f-ing up the waveform timing.
Those that like to experiment and want to make attenuators should be aware that the attenuator examples used a few posts earlier are intended for RF with 50 Ohm termination and WON'T work correctly with 75 Ohm digital signals and will actually provide a worse termination but could still be beneficial.
IF you intend to make those attenuators (or buy them but will have BNC or F-connectors mostly) make sure they are intended/calculated for 75 Ohm impedances.
Another thing... Some may hope to just pull out a multimeter and measure voltages or input/output impedances. In both cases this will not work and give VERY wrong indications in all cases as the real impedance is only valid for high frequencies and this usually is NOT the same as a DC resistance present in source and receiver side.
The audibility of it (or lack of) depends on many factors and cannot be predicted, it may be there, it may have no influence (unless limits on the receiver side are reached). HOW the improvements or deterioration manifests itself is also not predictable.
SPDIF is a serial signal that is always alternating. The average value (voltage level) of the electrical signal is always around 0V where the peak voltage levels can vary within certain limits (+/- xVolt). When the voltage amplitude(level) is too small errors will occur or the receiver won't operate or show drop-outs/ticks.
When the level is too high (usually due to improper termination = load impedance) an amplifier may also experience problems.
Within a certain voltage range all would be fine (level wise) as the 'decision point' is based on the middle of that voltage and needs a minimal voltage to be detected.
The fact that the average level of the signal is always (very close to) 0V over a minimal amount of time is very handy for engineers as the 'decision point' above and below one considers a certain voltage 'high' or 'low' is easily determined.
Also It is very handy to know that the 'high' and 'low' voltage levels do NOT represent the actual audio bits that are used by the DAC chip but are just a carrier. The bits are 'decoded' and the timing is used (in different ways for different DAC's) to synchronise with the transmitter 'frequency'.
When reflections, that are ALWAYS present but differ in amplitude (severity) depending on the termination on BOTH ends (transmitter and receiver side) as well as the properties of the used cable, when longer than 1 or 2 meters, distort that decision point jitter may be the result.
How much this influence is depends on sample (data) frequency, cable length and termination as well as output level of the 'transmitter' as the input range of the 'receiver' side.
For this reason it is VERY hard to predict HOW a certain combo 'works' and depending on the used DAC that jitter may or may NOT be affected by that jitter. That all depends on the immunity to jitter of the used DAC. This is determined by HOW the clock of the actual DAC CHIP used is (re)created.
When improper termination is present (hard to 'see' without specialised or home made test equipment) and the levels are within the level 'range' of the DAC all should be well.
In certain cases all that is needed is proper termination (but there is no way to really find out in which case without knowledge or test gear) is all that is needed.
the attenuation circuit may help reduce jitter IF the output voltage is high (of the source) and the receiver is sensitive enough to accept lower input levels due to A: better matching, B: jitter reduction by attenuation.
It also may help if the source level is too high and termination on the receiver end is wonky/bad and or the input of the receiver is 'over shouted' by the input level and reacts by f-ing up the waveform timing.
Those that like to experiment and want to make attenuators should be aware that the attenuator examples used a few posts earlier are intended for RF with 50 Ohm termination and WON'T work correctly with 75 Ohm digital signals and will actually provide a worse termination but could still be beneficial.
IF you intend to make those attenuators (or buy them but will have BNC or F-connectors mostly) make sure they are intended/calculated for 75 Ohm impedances.
Another thing... Some may hope to just pull out a multimeter and measure voltages or input/output impedances. In both cases this will not work and give VERY wrong indications in all cases as the real impedance is only valid for high frequencies and this usually is NOT the same as a DC resistance present in source and receiver side.
The audibility of it (or lack of) depends on many factors and cannot be predicted, it may be there, it may have no influence (unless limits on the receiver side are reached). HOW the improvements or deterioration manifests itself is also not predictable.