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Comment by ericcholis | Hacker News Reader

I was always impressed with how crisp remote guests or hosts often sound. Rather than sounding like they've called in, they sound like they're in-studio. Not terribly difficult to achieve, the remote person likely has a good audio setup and sends that recording to the engineer to mix together. Still, a nice touch.

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ISDN.

This is one of the last, best uses of ISDN. Guaranteed latency, ultra low jitter, and plenty of high-quality hardware purpose-built for getting the best possible studio audio over 2 bearer channels worth of capacity.

Indeed, this was also the original use case of ISDN before the internet.

ISDN still must be in heavy use in radio. Sirius XM has been using hardware IP codecs since the pandemic began, and once a three hour show you might hear garbled audio for a few seconds.

A lot of times, the remote guest visits an NPR station near them rather than visiting the location the show is being recorded.

In my experience the hardest part of this is syncing the remote audio files. For T>30 minutes the drift can be substantial.

Huh, I've consciously thought in the past of this as an outsider and concluded that by now it's a common enough task so of course they must've had an algorithm for doing it automatically.

Is there really nothing coming close to that?

There are plugins for different scenarios, but it turns into one of those problems where hearing and correcting issues is much easier for humans than computers. The tools available make it easier to fix problems, but it still takes a recording engineer to spot-check.

As someone who worked as an audio engineer, solving problems before they can occur saves so much time and headache. There's no reason to faff about with software or complexity-inducing algorithms when the whole problem can be fixed by toggling one switch.

Technically you could accomplish the same thing by applying a parametric eq to the master buss, but then you're no longer software agnostic.

It's like photography; sure one can post-process photos in photoshop. But getting everything right before taking the picture, at a hardware level, simplifies things for everyone involved.

unless they used a different sample rate, why would there be a drift?

And even if they had, there should not be any trouble resampling them into the correct rate for the project.

The clock drifts. Something needs to count those seconds. Even when the drift is small, phasing distortions become pretty obvious on lengthy recordings.

There's some interesting work going on in the AES to support synchronised audio over wide area networks, either through better recovery of PTP clocks distributed through WANs or using PTP with GNSS.

https://www.youtube.com/watch?v=tG7tCCKYDx4

Sample rates in audio hardware aren't like programming constants, where they're the same for everybody. Over 30 minutes, a 0.05% sample rate error gets you 1s of drift over the recording. As a reference, USB 2.0 has a 0.25% frequency tolerance (and is used to clock many audio devices).

Cheap quartz clocks in computers and some USB ADCs especially are prone to slightly changing their rates depending on temperature. So the sample rates can differ relative to each other.

Maybe actual clock differences? Not sure if that's the case, but in audio engineering, a separate clock may be used to keep all devices involved in-sync (many pro-level audio devices have a "clock" input for this very reason).

In RF engineering, it's typical to have all of your equipment referencing the same 10MHz clock (or a 1 pulse per second or IRIG-B). If I don't have a GPS receiver or a rubidium source, then I'll just pick the newest, most expensive piece of equipment with a built-in reference clock and fan it out to the rest of the equipment on the bench. Some portable spectrum analyzers have built-in GPS receivers so even out in the field you know you have a good reference.

I've wondered about this in long-form talk podcasts I listen to. I always just assumed there were audio file formats that included timecode.

Do you have any insights you can offer on how best to do this? I have to deal with drift issues on signal processing of .wav files, and I have always used a marker pulse every so often.

Is there a better way?

Interestingly one of the most enduring shows on NPR is Fresh Air with Terry Gross and she traditionally has not had her guests in the studio with her over her 40 plus years hosting the show. She has even spoken about how she's been able to use this to her advantage. This following is quick read on this:

https://www.inquirer.com/philly/entertainment/WHYY-NPR-Terry...

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