Hi,
Some years ago someone told me about particular techniques to show, with only a standard analog oscilloscope (30 Mhz), the distorsion, noise , dynamic range, frequency response, etc..
This without use a specific equipment like specturm analizer , distorsion meter , etc..
At the end, he confirm me that these technique are not applicable with a digital oscilloscope but only with analogue scope.
Now I don't remember exactly what are these technique.
Someone can suggest me some standard / advanced way to show the distortion, noise and other usuful informations with an oscilloscope ?
Some years ago someone told me about particular techniques to show, with only a standard analog oscilloscope (30 Mhz), the distorsion, noise , dynamic range, frequency response, etc..
This without use a specific equipment like specturm analizer , distorsion meter , etc..
At the end, he confirm me that these technique are not applicable with a digital oscilloscope but only with analogue scope.
Now I don't remember exactly what are these technique.
Someone can suggest me some standard / advanced way to show the distortion, noise and other usuful informations with an oscilloscope ?
I think the story is more subtle.
For instance, to look at distortion, obviously you need a source for a signal that has low distortion.
Then you need to see the difference between that signal and the amp output, properly scaled. Some scopes can show difference between two input signals but not to the accuracy needed for low distortion measurements.
Amp output level, same story: you need a source. Noise: can probably do with a scope, but not very accurate - you obviously need a scope with less noise than the amp.
So using just a scope for a variety of audio measurements is quite limited.
jan
For instance, to look at distortion, obviously you need a source for a signal that has low distortion.
Then you need to see the difference between that signal and the amp output, properly scaled. Some scopes can show difference between two input signals but not to the accuracy needed for low distortion measurements.
Amp output level, same story: you need a source. Noise: can probably do with a scope, but not very accurate - you obviously need a scope with less noise than the amp.
So using just a scope for a variety of audio measurements is quite limited.
jan
Thank you for your response 
My signal (squre, sine , triangle) can be done with my USB DAC (24 bit, 192Khz) 0.0015 THD (1 Khz)
I have all signals (250 files) at 24/196 Khz (from 20 to 20 khz).
For scope I was think something like hameg 407 (Analog scope) 40 Mhz, but I don't know sure if is sufficient for audio.
I was ask to many elelectonic enginers, they affirm that a good analog oscilloscope (like tektronix, hameg , philips , hp , etc..) is certanly better then all audio equipment in term of distortion , noise , etc.. my amp is a ARCAM FMj32.
However I'am not a very expert but someone time ago was show me (now I don't remember) different ways to show many audio informations with oscilloscope: noise , distorsion , dinamyc range, etc...
This not in terms of numeric measure (that is not possible) but with an analisys of signals.
There are some books of the arguments but I don't remember the name.
My signal (squre, sine , triangle) can be done with my USB DAC (24 bit, 192Khz) 0.0015 THD (1 Khz)
I have all signals (250 files) at 24/196 Khz (from 20 to 20 khz).
For scope I was think something like hameg 407 (Analog scope) 40 Mhz, but I don't know sure if is sufficient for audio.
I was ask to many elelectonic enginers, they affirm that a good analog oscilloscope (like tektronix, hameg , philips , hp , etc..) is certanly better then all audio equipment in term of distortion , noise , etc.. my amp is a ARCAM FMj32.
However I'am not a very expert but someone time ago was show me (now I don't remember) different ways to show many audio informations with oscilloscope: noise , distorsion , dinamyc range, etc...
This not in terms of numeric measure (that is not possible) but with an analisys of signals.
There are some books of the arguments but I don't remember the name.
I don't know exactly what you are looking for but a couple things come to mind. Assuming the scope has at least two channels to display. If you feed the test signal to one channel, and the output to the other channel, then adjust the two scope channels for the same size traces. Now invert one channel, and set the scope to ADD. If the two signals are exactly the same, no distortion, then the inverted should exactly cancel the non-inverted, leaving a trace at zero. Any difference between the input and output would result in the two not exactly cancelling, and the result would be a display on screen of the remaining distortion itself.
ANother technique would be to put the scope into X-Y mode. Put the test signal on the vertical, and the amp output on horizontal. If the signals are exactly the same, you get a slanted straight line on the screen. Adjust the amplitude or the scope sensitivity to make the line a 45 degree slant. If the output is not identical, any distortion or phase difference will result in the slanting line spreading into various shapes.
We used to use that method to look at stereo signals. If you applied left and right to X and Y axes, and fed it mono, you got a straight slanted line. But when the channel signal differed, you got a more chaotic picture. Also used that method in tape head alignment sometimes.
ANother technique would be to put the scope into X-Y mode. Put the test signal on the vertical, and the amp output on horizontal. If the signals are exactly the same, you get a slanted straight line on the screen. Adjust the amplitude or the scope sensitivity to make the line a 45 degree slant. If the output is not identical, any distortion or phase difference will result in the slanting line spreading into various shapes.
We used to use that method to look at stereo signals. If you applied left and right to X and Y axes, and fed it mono, you got a straight slanted line. But when the channel signal differed, you got a more chaotic picture. Also used that method in tape head alignment sometimes.
Hi Enzo, thank you for your response !!
These are good technique, I will do it both. What is the best for this measure analog or digital scope ? Or it is not very important ?
Can you suggest other type of measure (like show the noise for example) ?
and if I like to do some measure my DAC or CD players ? I have different wav file at 16 and 24 bit (square, sine , tringle)
These are good technique, I will do it both. What is the best for this measure analog or digital scope ? Or it is not very important ?
Can you suggest other type of measure (like show the noise for example) ?
and if I like to do some measure my DAC or CD players ? I have different wav file at 16 and 24 bit (square, sine , tringle)
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These are good techniques for measuring distortion maybe down to 1%, but certainly not better. The two channels gains will not be quite the same neither in amplitude nor in phase.
Looking at a single waveform you can probably eyeball a few % THD, IF you'r experienced.
So, your scope will show gross problems, but not the subtle ones.
Edit: most digital scopes have 8 bit vertical resolution which limits your accuracy anyway.
jan
The thing is that you can't see the difference on a scope between 2% and 0.5% distortion.
jan
My CD player and my DAC are 24 bit /196 Khz (I Listen Hi res flac).
The soundcard can't measure my source but is the opposite.
I have used a soundcard and True RTA with a mic for SPL meter to measure my speaker.
If you want a good result you must have at least a instrument better then the object that you want measure, otherwise there is no sense do it.
However I like to skip the distortion measure, and I like method to show the noise with oscilloscope of my amp.
There is way to do it ?
The soundcard can't measure my source but is the opposite.
I have used a soundcard and True RTA with a mic for SPL meter to measure my speaker.
If you want a good result you must have at least a instrument better then the object that you want measure, otherwise there is no sense do it.
However I like to skip the distortion measure, and I like method to show the noise with oscilloscope of my amp.
There is way to do it ?
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