[UVB-76]

Optical just feels like a cleaner interface. USB requires drivers, and introduces timing and electrical interference issues.

USB also feels like overkill for an audio interface. It allows transfer of data in both directions, and introduces security risks if you don't fully trust the DAC you are connecting to, or worse still if the DAC is connected to a network/the Internet.

I'm not suggesting for a moment there is any audible difference between the two, but optical audio has always struck me as a 'cleaner' way to output digital audio.

I would be extremely annoyed if it were removed from the MacBook Pro.

[Someone]

"USB requires drivers, and introduces timing and electrical interference issues"

On multi-tasking OSes with memory protection, everything that accesses hardware requires drivers; it's 'just' the amount of driver code that differs (which, for USB vs optical, probably is quite a big difference)

Also, originally USB didn't support isochronous channels at all, so one couldn't guarantee that one could send audio out continuously.

[driverdan]

Most USB DACs do not require special drivers. The drivers are included with the OS. If you don't trust the device to be plugged in via USB then you shouldn't buy it. Most DACs are fairly simple hardware and tear downs are common. Check Head-Fi and other audiophile sites.

[mbell]

> introduces timing

SPDIF also combines the clock and data in a single signal, it has more or less the same issues with jitter that USB does. Not that it matters since I'm not aware of a single proper study that indicates jitter is audible.

[DigitalJack]

I would be shocked if there wasn't a hardware fifo in that receiver path.

[mbell]

Depends on the device.

Often there is a receiver IC that will perform clock recovery and convert from S/PDIF to an internal digital audio format, I2S most commonly, to send it to a DAC IC. In this simple setup there generally is no more buffering than needed for operation of the pieces.

In more complex situations, either where the S/PDIF signal is being re-clocked or where the data on the S/PDIF line needs to be decoded, then they will be buffering.

To clarify some terms:

Re-clocking - the system detects the clock rate of the signal but does not use that clock as the reference, instead it will generate it's own clock at the proper frequency. Some buffering is needed here to offset drift between the clocks causing sample starvation.

Encoded data - S/PDIF (really AES3 for the most part) has a framed block transmission format not entirely dissimilar to a TCP packet. There is meta-data, frames, sub-frames, etc, it's not just 'pure audio'. Various formats can be stuffed into these data frames, like compressed audio data (5.1 Dolby Digital for example).

[efraim]

USB seems a bit overkill but since it's already there isn't it more overkill to add a dedicated optical port for digital audio?

[tylerjd]

The digital optical port is baked into the 3.5mm jack on most upper-end laptops and Macs. Though sadly my MacBook Air does not have one.

[mbell]

The port isn't the important bit that is added, it's the supporting circuitry and photo diode to drive it that adds complexity to the design.

[DigitalJack]

And the little physical switch in the port that broke on my last MBP. My port got stuck in optical mode.

[kennell]

Mine got stuck serveral times too. Was alway able to fix it with a toothpick.