Thanks Brian, for the thorough explanation. Kind regards, António SintraWorks
> On 20 Aug 2017, at 22:03, Brian Willoughby <[email protected]> wrote: > > The issue you describe is not specific to CoreAudio. It is the nature of > digital audio recording whenever multiple devices are used. > > Unless you connect Word Clock or some other digitally clocked signal between > two or more devices, there will always be some drift between them. It is > impossible for two or more digital sample clocks to remain sample accurate > over a long period of time unless those clocks are connected electrically (or > optically). Your Mac or iOS device, just like any digital audio device, each > has a crystal oscillator that is the master clock. Crystals are very > accurate, but no two of them precisely match (and even if they did, their > clocks would not necessarily happen at the exact same instant). So, even if > you set both devices to 48 kHz sample rate, the samples will not be the same > when separate recordings are made on the two devices. The analog waveforms > will match fairly closely, but the digital samples could be vastly different. > > In recording studios, and even in advanced mobile recording setups, there are > multiple technologies to deal with this issue. One is Word Clock. With Word > Clock, a master clock can be connected to all slave devices, and if > everything is set correctly then their samples will all be aligned no matter > how long the recording. Other options include AES3 digital audio, ADAT, and a > few others. These latter connections pass audio as well as clock, and may be > more or less accurate (in terms of clock jitter) compared to Word Clock > technology. > > Bottom line: this is not an issue with RemoteIO. > > Brian Willoughby > Sound Consulting > > > On Aug 20, 2017, at 12:11 PM, Antonio Nunes <[email protected]> wrote: >> On some software I’ve been writing, I use remoteIO to playback very short >> sound files. In the rendering callback I provide the required samples. This >> is all working well and playback sounds great. >> >> The only issue I have is that playback appears to be somewhat inaccurate, in >> the sense that it is overall a tad too quick. The thing is, I also have the >> ability for a playback session to be recorded live to disk. When I inspect >> the resulting sound file in a sound editor, I see perfect results. Lets say >> I set everything up for a bleep to play 60 times a minute and I start >> playback with live recording to audio file on. I also record the playback >> session through a microphone on another device (my Mac). After the session >> the file recorded through micorophone is trimmed so that the first bleep >> starts a 0 time. >> >> When I subsequently inspect both files in a sound editor I see that the file >> recorded file has each sample start exactly on time at every second advance, >> throughout the whole file. In the file recorded through microphone though, I >> can see that each subsequent bleep starts ever so slightly too early, >> resulting in an accumulating discrepancy. >> >> I haven’t been able to find a clue as to why the externally recorded sound >> file doesn’t show equal results to the internally recorded sound file. When >> I playback and simultaneously record through microphone the sample perfect >> sound file, the results are as expected, without accumulating error. My >> guess is that maybe remote IO is playing back the samples ever so slightly >> too fast. >> >> In fact…as I was writing this, I realised I could also playback the >> internally recorded sound file on my iOS device and record it via mike on my >> Mac. When I do that, I see a very similar discrepancy to what I see when I >> record the live session via mike on my Mac. So: could it be that remoteIO’s >> actually playback speed is slightly inaccurate, where samples are played >> back a fraction of a millisecond too fast, resulting in an accumulating >> error? >> _______________________________________________ Do not post admin requests to the list. They will be ignored. Coreaudio-api mailing list ([email protected]) Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/coreaudio-api/archive%40mail-archive.com This email sent to [email protected]
