When you installed your first Wheatstone Blade, you were asked to select a sample rate for that Blade. You either chose 48000 samples/second, or 44100 samples/second. As you installed additional Blades, you presumably selected the same sample rate for each subsequent installation.
The sample rate, of course, denotes how may times an analog audio recording is sampled per second of audio as it is being converted to a digital recording or audio stream. We generally consider the 44.1kHz sample rate more than sufficient for radio broadcasting because it can reproduce audio frequencies up to half the sample rate, or 22050 Hz, which is considered a higher frequency than most human beings can hear.
So why would you even need to consider a higher sample rate, such as 48 kHz, which is also offered in Wheatnet? Well, hi-fi enthusiasts will quote several reasons ranging from anti-aliasing issues to bone-conduction (through which we are supposedly able to hear even higher frequencies than we can hear through the air), but we are more concerned with a couple of practical reasons.
- TV stations have traditionally selected 48 kHz as their sample rate as it is mathematically easier to sync audio with video (since 48 is evenly divisible by 24, a common frame rate for video). If your radio station needs to share real-time audio with a TV station this make things a bit easier.
- AES-67 specifies the use of 48 kHz as a required sample rate for audio conforming to its published protocol. AES-67 allows interoperability of devices utilizing Audio Over IP for transport. Since most companies with AoIP protocols are proprietary (Wheatnet, Livewire, Ravenna, Dante), without an independent protocol to enable sharing of audio signals, each would have to live in its own world and there would be no way to share audio across these networks.
Wheatnet systems allow you to choose 44.1 kHz or 48 kHz when you set up a blade, but the system sample rate is actually controlled by the Clock Master blade, and it gets its marching orders from Navigator.
You can change the sample rate for your system on this pane in the System | Info section of Navigator. Here you can see the system sample rate set to 48 kHz (if any blades were initially set up as 44.1, they would have been changed by this setting). It’s also possible to clock your system from an external source, such as one of the other proprietary AoIP protocols mentioned above. If you choose to do this, our internal clock (the metronome) will sync to that external source, and all of the blades will still observe our metronome (now synched to the external input) for their clocking needs.
If you are going to be interfacing AES67 devices with Wheatnet, and creating 1ms AES67 streams within Wheatnet to send to other AES67-compatible devices, you will need a PTP Grandmaster clock. This same Clock Master screen is where you will select PTP as your Primary External Reference at which point you should see the GMID of your clock appear, the green PTP Status light come on and a message that you are locked to the PTP clock source. See this article for more on setting up a PTP clock.
AES67 does not preclude the use of sample rates other than 48 kHz. It also supports 44.1 kHz and 96 kHz, but the protocol seems to be focused on 48 kHz, 24-bit audio with 1 ms packet timing. All conforming devices are required to support this sample rate, bit depth and timing so if you use these settings you should be able to interface easily with any other AES67-compatible device.
If you are using the Wheatnet PC driver, you will want to check the sample rates in Windows for each driver channel (both playback and record) as these need to match the sample rate of your Wheatnet Blade system. If the sample rate is higher than you Blade sample rate, your audio will play back slow. If it’s lower than your system sample rate, it will play back too fast. It doesn’t matter what sample rate your audio files were recorded at, as Windows does a good job of resampling those when necessary. But keep an eye on these settings:
Microsoft has a bad habit of changing these settings when Windows updates are performed. Not every time, but seemingly at random times. If your audio is suddenly recording or playing back at the wrong speed, you probably just suffered from Patch Tuesday Syndrome and need to check these settings.