CM108 and PCM2704 USB audio dongle - Page 1
Forum section: electronics / projects

author: andymouse
author_rank: Contributor
timestamp: February 27, 2022, 09:15:10 pm
content: I am taking my first steps in the digital audio world and I have two dongles from eBay, I am using Audacity to supply a 1kHz sine wave at about 300mV RMS and observing the output on a scope, what I see is a nice sine from the PCM2704 based device, the same signal from the CM108 has clear 'steps' in it with a frequency of 98KHz, I noted that both devices have the same sample rate and clock frequency and the same amount of bit's (16) and I am wondering why, closer examination of the PCM2704 sine shows the same steps but much higher frequency and barely visible, I have a basic understanding that the DAC outputs a voltage proportional to a digital number and need to do further reading and I understand that there are two common sample rates used 44kHz and 48kHz, I would like to know why the two sine's are so radically different please, they are like day and night, what route should I take to understand what I am seeing please.ThanksAndy

author: magic
author_rank: Super Contributor
timestamp: February 27, 2022, 09:30:18 pm
content: Good audio DACs contain reconstruction filters which attenuate frequencies above Nyquist and thus eliminate the steps effect, leaving a clean sinewave.Most audio DACs have fewer bits of resolution than advertised and make up for it by oversampling - each sample is replaced with a sequence of samples at much higher frequency so that additional bits of resolution are gained by averaging. Oversampling also enables simpler reconstruction filters.The CM108 looks like a crappy DAC that internally oversamples at some random (and rather low) frequency of 98kHz and skimps on filtering.

author: moffy
author_rank: Super Contributor
timestamp: February 27, 2022, 09:47:12 pm
content: Are you sure you are not just observing the steps of the 8 bit ADC of your scope?

author: andymouse
author_rank: Contributor
timestamp: February 27, 2022, 10:05:57 pm
content: I don't think so as all I change is the device, no settings change, one has a clean sine and the other has large steps, I am beginning to think it's a rubbish DAC it cost 8 quid and seem to be all over ebay it has the two 3.5 mm jacks (pink,green) and the USB is formed from the PCB itself however it does look well made, the good one (PCM2704) is manufactured by Burr Brown, usually great chips, I have another even cheaper one that also has a good looking sine and thats a 'blob' chip I am trying to learn how these things workCheersAndy

author: andymouse
author_rank: Contributor
timestamp: February 27, 2022, 10:10:13 pm
content: Thanks, I shall look at 'oversampling'

author: andymouse
author_rank: Contributor
timestamp: February 28, 2022, 12:58:25 am
content: Interestingly the PCM2704 mentions that it has oversampling being 128 fs not entirely sure what it means but it sounds like what you described, however there is 'no' mention of any sort of oversampling mentioned in the datasheet of the CM108 so it woul appear that that explains the crappy waveform...what do you think please am I on the right track ?CheersAndy

author: Bassman59
author_rank: Super Contributor
timestamp: February 28, 2022, 03:12:23 am
content: Quote from: andymouse on February 28, 2022, 12:58:25 amAll modern audio converters use oversampling. How it is implemented separates the good from the bad.

author: magic
author_rank: Super Contributor
timestamp: February 28, 2022, 07:11:10 am
content: Quote from: andymouse on February 28, 2022, 12:58:25 amFs is the input sampling frequency (44.1/48kHz) and 128fs is simply 128x more, so a few MHz