What are the advantages of using an optical audio port?

[Spectrox_]

I bought a Fosi Q5 DAC a few months back and I noticed it had an optical audio io; what are the advantages/disadvantages of using an optical audio io? How are your personal experiences using one? Do you notice a difference over using RCA or USB?

[Dedayog]

RCA is analog (I assume you mean the red/white wires) while USB and Optical are digital.

 

I'd go USB or Optical, no real benefit of one over the other.  

 

[suedseefrucht]

37 minutes ago, Spectrox_ said:

what are the advantages/disadvantages of using an optical audio io?

It's a digital way to transport audio. So it has a limited resolution. But also analog noise has no influence on the signal quality. Also you can transfer 5.1 sound with only 1 cable. 

[mariushm]

Optical output means the audio chip sends the uncompressed digital audio over the optical cable to a receiver which may or may not have better DACs  (digital to analogue converters) to convert the digital signal to analogue one, then amplify the signal to speakers or headphones.

Basically, instead of using the DACs built into the chip (or separate DACs on the motherboard), you may be able to get better conversion inside a device further away from your computer. The inside of the computer can be noisy, with electric noise radiated by the cpu and video card VRMs into the air and through the motherboard, and that in theory can affect the sound quality. 

 

5.1 can be sent over one single optical cable, but depending on standard, it may be compressed 5.1, so a bit lossy.

 

USB is also digital, the digital sound generated and mixed by Windows is sent to the usb sound card driver which sends it in lossless format to the usb controller at the other end of the usb cable. The controller then uses DACs and amplifiers to produce sound.

So same concept as optical out to a audio device.... but with usb, due to how data is formatted in data packets and sent in a specific way through the usb cable, there is potential of having some jitter, but in practice it's not an issue, and even if it would occur you wouldn't notice it.

[Spectrox_]

That's exactly what I was looking for! Thank you for the detailed response; you really made it easy to understand.

One question though with what you said below,

37 minutes ago, mariushm said:

but with usb, due to how data is formatted in data packets and sent in a specific way through the usb cable, there is potential of having some jitter, but in practice it's not an issue, and even if it would occur you wouldn't notice it.

Would different generations of USB such as 2.1, 3.0, 3.1 etc.. make any difference when it comes to how the data is formatted and sent through the cable? Perhaps less potential for jitter due to the bandwidth capabilities of a higher generation USB port? Or does bandwidth even affect how the audio is transmitted through the cable?

Edited September 16, 2021 by Spectrox_
Elaborated

[mariushm]

No.  It's not a bandwidth issue. Audio data is not that big ... ex 24 bit audio, 192 kHz , stereo is  2 (channels ) x 192000 (samples per second)  x 3 (bytes, 3 x 8 = 24 bit per sample)  = 1,152,000 bytes per second or 1.1 MB/s.  For 48 kHz, we're looking at 288,000 bytes per second or 280 KB/s  

And most people would use 16-24 bit 48kHz  (there's little benefit to using 96 kHz or 192kHz) and maybe 5.1 ... that's 6 x 3 bytes x 48000 = 864,000 bytes/s

 

USB 2 can do 480 mbps , so there's plenty of bandwidth.

 

Decent USB DACs will have retimers so you wouldn't notice any jitter.

 

The original Toslink / optical out supported maximum 3.1 mbps (around 400 KB/s) and 20/24 bit audio at 48 kHz, that's one of the reasons why only stereo output is uncompressed and 5.1 or higher had to be compressed with Dolby Digital Live / DTS.

 

[Spuriae]

1 hour ago, mariushm said:

So same concept as optical out to a audio device.... but with usb, due to how data is formatted in data packets and sent in a specific way through the usb cable, there is potential of having some jitter, but in practice it's not an issue, and even if it would occur you wouldn't notice it.

PCM has jitter too, based on the source clock and skewing. USB must be converted to PCM so jitter is based on the downstream device's clock.

 

If the upstream device has better clocking use Coax; if the DAC is higher quality than the source use USB; if there's a ground loop try optical. In practice the differences are unlikely to be audible.

 

The Q5 uses a CM6642 for USB to PCM, which isn't great. It's still probably lower jitter than the Realtek output if you were planning to use your PC as a source. Either way, you're very unlikely to be able to hear any difference unless the optical IO happens to fix a ground loop. Personally I'd stick with USB.

[ShearMe]

On 9/16/2021 at 2:42 PM, mariushm said:

The original Toslink / optical out supported maximum 3.1 mbps (around 400 KB/s) and 20/24 bit audio at 48 kHz, that's one of the reasons why only stereo output is uncompressed and 5.1 or higher had to be compressed with Dolby Digital Live / DTS.

 

I've never understood why electronics manufacturers havent just switched to ADAT instead of S/PDIF

[Spuriae]

37 minutes ago, ShearMe said:

I've never understood why electronics manufacturers havent just switched to ADAT instead of S/PDIF

PCM-based protocols (SPDIF, I2S) will always be around because they're so much more cost efficient and easier to design for. You can run a DAC using nothing but glue logic on those; for anything more complicated a microprocessor and clock regeneration are usually required.

 

Same reason SPI for embedded systems isn't going away anytime soon.