AERO processors that have SDI and AES-3 I/O baseband I/O have different clock options then AERO processors which have AoIP (AES67/SMPTE 2110-30/31 I/O. The clock options for these processor types will be shown separately below.
AERO processors with SDI and AES-3 I/O
AERO processors may receive and send digital audio to and from multiple sources. When all of the audio and video sources do not share a single master timing source the audio data will come into the AERO processor at different clock rates. This is very bad!
For example, audio packets are coming from the news studio console at the rate of 1000 packets per second according to the studio console’s internal clock. The AERO, running on its internal clock, only reads 999 packets in the same amount of time. The length of 1 second is literally different in the console than in the AERO.
Every independently clocked digital audio device will have a slightly different internal clock rate. Some devices may send and receive 1001 audio packets in the same time that another device sends and receives 999 packets. The result is that over some number of seconds a device will not have an entire audio packet to send when a receiving device needs to receive it. No matter how big the send and receive buffers of the devices are, each device will eventually have to skip packets or repeat a packets or perform a clock reset. This causes pops, clicks, dropouts and other audible artifacts. This is why a single timing source for AES-3, AES67/SMPTE2110-30 audio and all video sources MUST SHARE A MASTER CLOCK.
Having described why every facility should have a master clock source, sometimes facilities do not. AERO processors with SDI inputs have a workaround built in. If an SDI source connected to an AERO is selected as the primary sync reference then AERO processors will derive audio timing from the SDI signal itself. It will de-embed audio from the SDI source, and embed audio into the SDI output at the video’s clock rate. If SRC (sample rate conversion) is selected for each AES-3 input and output then these audio I/O will be sample rate converted so they are in sync with the SDI audio.
Note: Dolby Digital (AC-3), Dolby Digital Plus, Dolby AC-4, Dolby E, Dolby ED2 and other bitrate reduced formats cannot be passed through SRC. An SRC will destroy the coded audio.
A Vref (video reference signal) may be connected to AERO processors that have baseband I/O.. In this case it should selected as the primary reference and SDI as the secondary reference. Secondary sync sources may be selected in AERO.20, 200 and 2400. If the Vref signal should disappear the SDI input will replace it as the reference.
Some facilities will not connect SDI to their AERO processors. Only AES-3 and/or AES67/SMPTE2110-30 audio I/O is used. In this case AES-3 input 1 may be chosen as the sync source. Alternately, a silent AES-3 stream or one with audio, can be connected to the AES-3 Ref input. Whichever AES-3 clock source is selected, that source must be active 24/7/365. A switched AES-3 source is NOT suitable as a timing reference.
The AEROs also have an Internal 48KHz clock source for audio. In the absence of a master clock this internal clock may be selected. All audio inputs must use SRC to minimize audio timing errors. Downstream devices must have input SRC to minimize audio issues in that audio receiving device. (Please see Note above, regarding coded audio and SRC)
AERO Sync Inputs
SDI Input #1
AES3 Input #1
AES3 Input #4
AES3 Reference Input
Analog VRef
Internal 48 kHz oscillator
Below is an example for audio reference clock setting in AERO.20, 200 and 2400.
Navigate to: System > Baseband I/O > Main to clock sources.
Figure 1: Clock status and configuration drop-down menus (Main page)
Reference Clock Indicator
NfRemote has a reference clock indicator. This indicator is Linear Acoustic logo. It is located in the top left corner of the NfRemote window. The indicator uses color to denote the status of the reference clock. It will also indicate a fault in communication between AERO hardware’s baseband I/O card and the system processor.
Logo is solid gray: AERO is operating normally and is using the primary reference clock.
Figure 2: Primary baseband reference clock is in use
Flashing yellow: The AERO is operating normally, but the primary reference clock source has been lost, and AERO is using the secondary reference clock.
Figure 3: Secondary baseband audio reference clock is in use
Flashing red: One of three conditions has occurred: The AERO has lost both the primary and secondary reference clocks. (It will be using the internal 48 kHz clock). Or, there is a failure communicating with the baseband I/O card, or there is a failure of the baseband I/O card itself.
Figure 4: Baseband audio reference clocks are missing, internal oscillator in use
PTP Clock
The AERO.20, 200 and 2400 add a new level of complexity to synchronizing the timing of these units. In addition to considering SDI clock and AES-3 clock these processors add PTP clock, for AoIP I/O, to the mix. AES67/SMPTE 2110 I/O requires a PTP (precision time protocol) clock signal to be on the AoIP network. AoIP I/O MUST be synchronized with the PTP master clock on the AoIP network. In a facility that does not have a master time reference that is used to synchronize SDI, AES-3, and AoIP, it is critical that the AEROs AoIP SRCs are turned on for any active audio I/O. (the exception is coded audio as defined in SMPTE 2110-31) This will help to ensure that AoIP I/O streams routed through AERO processors are free of audible artifacts.
The status of the PTP clock can be found here: System > System > AoIP > Clock.
Figure 5: AoIP Clock configuration page
The PTP status page shows:
The current PTP clock is valid and within proscribed parameters. It is shown in green text
Primary and secondary clock priority of the received clock
Clock Class
PTP master clock IP and the closest boundary clock IP
Grand Master PTP Clock ID, identified by its network MAC address
Nearest boundary clock ID, identified by its network MAC address
Sample Rate Conversion
In general, PCM audio should pass through AERO SRCs to synchronize their timing with the AERO processor’s assigned sync source. As shown in a Note above, bitrate reduced audio such as Dolby or MPEG encoded audio, cannot not pass through SRCs without being destroyed.