Electronic's scope and audio - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: January 22, 2023, 12:31:24 pm
content: So I wanted to see the noise on my latest audio project. It's an optical -> i2s interface and a PCM5102 DAC. Powered from a Tenma bench supply on 3.3V.Scope is connected to the right channel audio out of the DAC. The DAC is receiving 0s. It's not in shutdown and it's not idle. It's running and playing silence.I set the scope up AC x10 probe (although x1 would probably do). I set the vertical to close to max, so see the trace and set the horizontal to cover about 2 seconds of audio, out far enough to get slow acquire running.The scope was showing a very distinct pattern of the I2S master clock at 24.576Mhz and the bit clock around 3MHz with about a 100mVpp.Then I flipped to the FFT Maths mode and as expected there were two very large spikes at those frequencies. The noise floor along the bottom was really far below.I'm not really (at this stage) worried about the out of band clock noise, so I tuned the FFT in to show just the first 35KHz.FFT is attached below.To me it looks like what I should expect. The headline figure on the DAC is -112db SNR and range. Having a floor show up at -100dbm seems fair enough to me. However I might be reading it wrong. Sure there are a few spikes in the band, but the highest is -53db.Later (no capture) I removed the audio circuit I was using which included a mains reference somewhere and connected the DAC up to a battery powered headphone directly and the scope wouldn't even capture on the noise. I had to turn off HF Reject and have the scope capture on the higher frequency clock noise and then use the FFT to look at the noise. The floor didn't fall much but the spikes all dropped or disappeared.I was however stupid enough to ask if the trace looks normal on an audiophile forum. Oh boy. The first thing is, they don't seem to understand anything about higher frequencies. They queried why I was sampling at f=60Mhz for example. I tried to explain that is the frequency of whatever it's triggering on or something, the sample rate in this case was 1.0MSa/s as stated. Then they start with the 8 bit resolution and while one or two of them are smart enough to realise the size of the bits changes with the front end gain, most just don't.What surprised me even more is them claiming that a basic off the shelf laptop or PC soundcard chip actually has 24bit resolution. At line level that's like 89nV. I find it extremely unlikely that a PC sound card IC that costs about £3.50 has an ADC circuit in it capable of accurately measuring discrete voltage differentials of 90nV across a range of 1Vpp. ... and in addition has the analouge front end capable of that same range. I highly doubt its got 24bit accuracy, 24bit resolution maybe.They also seem to think I'm trying to characterise a DAC or ADC. Rather than the whole circuit noise at the point it's important.Until someone proves to me otherwise, the reasons for >16bits and >48K are not for the analogue consumption side, they are for the internal mathematical and floating point approximation problems... and headroom. 24 and 32 bit >48K post analogue LPF filter is for unicorns.On those audio forums, there are a few who know what they are doing. They are the real enigma though. They are smart enough to understand what they are doing, but yet, still dumb enough to continue chasing a perfection level far beyond what any human can hear. Yet they continue to do it. It's like successful physicists who are deeply religious. Doesn't compute. The vast majority of them are parroting idiot sheep though. Sheep with far, far too much money to waste.

author: jonpaul
author_rank: Super Contributor
timestamp: January 23, 2023, 12:17:53 pm
content: Impossible to comment with lacking full information. Measurement of noise requires good setup, correct probes, and elimination of EMI that contaminats the measurements.exact circuit schematic, PCB photo ?Probes type, settings of scope, legnth of probe ground return circuit ?Scope noise floor, BW?jon

author: paulca
author_rank: Super Contributor
timestamp: January 23, 2023, 12:46:49 pm
content: Quote from: jonpaul on January 23, 2023, 12:17:53 pmI'm not characterising anything. The EMI and the nextdoor neighbours cat farting are considered noise. Excluding the noise to measure the noise is fine if you are writing a datasheet.All I want to know is what am I amplifying in the next stage and if there is anything I should be greatly concerned with.The ground of the scope is connected to the same ground as the amplifier and the DAC is feeding a 10KOhm line in.I believe this is the fairly cryptic noise figure from the SDS1104 datasheet.ST-DEV ≤0.1 division (≥2 mV/div)Probe is a standard 100Mhz x10 probe.Breadboard contains an Optical SPDIF +PCM1921?? interface board emitting I2S and a PCM5102 DAC receiving. Signal is all 0s.Outside the audio band the scope reveals a lot of clock noise. It also reveals the I2S signals are about 4.2V peak to peak, on a 3.3V rail, which is probably the fault of the breadboard. It's usually a lot worse but the Optical/I2S interface board has got an H74 series transceiver or buffer on the I2S lines with series resistors.I get that, if you were characterising the circuit itself or characterising a component within it, that isolating variables so you can give a good measurement of specific things is important.What I'm trying to do here is "measuring the circuit in test", which includes the next door neighbours atomic farts and the power line noise, the PC noise, the earth noise, the clock noise, everything that will be in the circuit when I am using it. More importantly what is going to be gained linear x10 and sent to headphones or speakers.I realise a true electronics noise engineer would not stop and would not sleep until he has isolated each and everyone of those using controls, multiple measurements, statistical anaysis etc.I just want to know, am I amplifying a hum from somewhere that will annoy me.I honestly thought 100dbm w/ anomolies at -59 "sounded about right" and when gained 10x it still not really going to me of much concern as it will still be way below -40db.As you point out, it's a complex subject and my measurement on the FFT might be completely bogus.

author: jonpaul
author_rank: Super Contributor
timestamp: January 23, 2023, 12:56:44 pm
content: Bonjour cher plulca;In epoch 1980s..1990s we consulted and designed digital audio interfaces for transmission with ADC/DAC IC, high end hifi and broadcast firms.I was on the AES standards committee for suc as AES/EBU. AES3id, SPDF. very familiar with the issues....I would check the specs of the PHY TX/RX of these interfaces. Like RS-485, Ethernet, AES/EBU, I2S is aa digital signal with wide margin for noise.The RX may have hysteretic Schmidt triggers and/ or differential amps.So, the noise you are seeing may not be affecting the accurate digital signal reproduction.Note, an an all 0s test should be augmented with a PRN and 1010 test as behavior of the PHY RX and decodes will be different for different test patterns.By the way, optical DA (TOSlink) was started in 1980s and the latest models made like Toshiba TORTX 176 were not consistent mate to mate and rather jittery digital outputs. .Highest quality DA nowadays is either AES/EBU, AES 3id or vai Ethernet AES067, the TOSlink optical is a legacy of the consumer grade equipment.Have an ABSOLUTELY fantastic day!Jon

author: paulca
author_rank: Super Contributor
timestamp: January 23, 2023, 01:32:20 pm
content: Quite interesting.I like the idea of the 1010 and similar bit pattern tests. Anywhere in the first 8 LSB bits. It might not be 24bit accurate, but it should respond to small bit values in some way. Not that that level of precision is in my target area. I'm only going to 24bit for processing needs, though if I can process incoming 24bit rather than up cast 16, thats even better.The clock noise very likely isn't coming from the DAC output per-sae, it's more likely just induced noise because they are square waving away in circuit.I'm not that concerned with audiophile level accuracy, jitter and drift are fine, I'm accepting them. I'm more interested in high functionality, high flexibility, modularity rather than outright quality.This was pretty much confirmed to me, as on Friday I was listening to this setup on headphones for a good few hours and I was admiring how nice it sounded and wondering if that was because I was (I thought) running it at 192K, 24Bit.It was only later that day when I scoped it digital side did I realise the optical spdif controls the bit depth and rate and the settings I was fiddling with where for the optional ADC on the board. So I was listening to it all along at 16bit 48K.Regardless of whether an electronics scope can detect the accuracy of a DAC, I'm 100% certain my ears can't.

author: paulca
author_rank: Super Contributor
timestamp: January 23, 2023, 01:35:23 pm
content: Speaking of that ADC... I see an opportunity for some ADC/DAC torture. Feed them back into each other and give them a polite pulse to set them off. Sit back and watch.I've been listening to too much "The Orb".

author: jonpaul
author_rank: Super Contributor
timestamp: January 23, 2023, 04:21:52 pm
content: Rebonjour cher Paul: BRAVO for your interest and work in digital audio.0/ ref on I2S PHY waves, noise etchttps://prodigytechno.com/i2s-protocol/1/ Our old friends at Audio Precision (EX TEK AA501 SG505) have the industry std on digital/audio testing. Many fine app notes re noise, digital interfaces.https://www.ap.com/audio-analyzers/For details re Noise, # bits, etc see the excellent app notes and AES papers esp of Dr Julian DUNN (RIP).2/ Typical I2S spechttps://www.infineon.com/dgdl/Infineon-Component_I2S_V2.70-Software+Module+Datasheets-v02_07-EN.pdf?fileId=8ac78c8c7d0d8da4017d0ea3d6f826043/ ADC/DAC ICs have lots on noise, dyn range, FS, see mfg spec sheers and app notes:TI (Burr Brown)AKMESSAnalog DevicesCirrus LogicHappy researching!Jon(PS: old connexcions at IC firmssince 1980s,our transformers are on many of the mfg eval bds. )