Who is using external clock with DaVinci Resolve? How?

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Michael Holmes

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Who is using external clock with DaVinci Resolve? How?

PostThu Feb 22, 2018 9:37 pm

I have been talking with BM Support, trying to understand how I can use an external clock with DaVinci Resolve. We currently mix audio in Pro Tools HD, using a high-end Antelope Audio word clock (via USB to the Windows computer). This is easy to set-up in Pro Tools.

This clock gives use a very solid, stable audio mix out of Pro Tools back into DaVinci Resolve.
I want to use the same clock with DaVinci Resolve to maintain the audio quality we get out of Pro Tools, not an "internal clock".

What I have been told is:
- There is no way to use my clock connected to the Windows computer, as I can do with Pro Tools HD.
- I need to buy both (1) the Fairlight Accelerator PCIe card and (2) the Fairlight Audio Interface. And it appears that, even then, I cannot use my Antelope Audio clock, but would be dependent on the Fairlight clock in their Audio Interface.

Is anyone using an external clock with DaVinci Resolve? My background is audio mixing/mastering, and all the good mixing studios I know use a high-end external clock. If you are not, how do you ensure you have good sample rate clocking?

Is anyone using this Fairlight gear? I have found no way to reach a Fairlight tech person who can explain this gear in detail. I have no idea how good the clock in the Fairlight Interface is; and in any case, I would prefer to use the clock I already own.

Thanks!
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Andrew Kolakowski

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Re: Who is using external clock with DaVinci Resolve? How?

PostFri Feb 23, 2018 3:36 pm

Don't know much about audio mixing, but I would say: stick to proper tools for this. Resolve is 1 for all approach and has many shortcuts in many places (as otherwise it's impossible to be 1 for all tool). Resolve is not best NLE, not best grading tool neither mixing app- for this very reason of trying to be all of them.
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Reynaud Venter

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Re: Who is using external clock with DaVinci Resolve? How?

PostFri Feb 23, 2018 4:39 pm

Greetings Michael,

As previously discussed via DM, the monitoring DAC locks to the incoming clock signal via the digital stream. There is no need for an external clock, many DACs do not even provide a Word Clock Input because they clock to the incoming digital stream.

Here's Dan Lavry's take on using dedicated external clocks:
“In TapeOp I presented a condensed summery from a technical standpoint. I explained that internal clocking is made up of one oscillator circuit (hopefully a very good one) located in the AD chassis near the converter. Using external clocking calls for two oscillators, one in the AD and the other in the external clock box, and then one needs to add circuitry inside the AD chassis to lock the 2 clocks together (Phase lock loop or PLL circuits) *or digital synthesis. That plus a cable connection and issues such as separate chassis grounds causes an increase in jitter. So why would locating an oscillator in a separate remote chassis be better? Generally speaking, the opposite is true.

Some try to argue that an external clock may improve an AD with a poor internal clock. While anything can happen, this is a very unlikely scenario. For that to happen, the AD designer would have to come up with a poor internal fixed oscillator circuit, and within the same design have a much better second oscillator. Even if such an unlikely thing happens, there is still the cable issue (electromagnetic interference), the grounding issues (ground potential differences and ground current) and of course the PLL circuit.

There is even stronger reason why a single internal oscillator is better, but it takes some technical expertise to fully understand it. The short of it is: The AD conversion process takes much more than a simple word clock. A word clock ticks over once per sample, but there are many bits per sample, so a second clock (such as at 32 times word clock speed) is needed. Also many designs call for an additional much higher frequency clock. For example, a 64fs oversampling converter, or a 1024fs call for a very high speed clock. Now, which one of the 2-3 clocks is the critical one? As a rule, it is not the word clock!

Internal clock design enables for much better handling of jitter. The internal clock provides low jitter at the higher frequencies where it is needed. When using an external clocking box, the “connection” to the AD circuit is indirect. The word clock has to be multiplied to a faster rate, going through additional circuits, and with it, the jitter also multiplies and grows.

There are times that we need to use external clocks, such as for synchronization of multiple chassis. If it is needed, use it, and when you can, use converters that handle external clocking well. But the notion that external clocking can help improve conversion is flawed. The opposite is true, and in the real world, it is very unlikely.

As a rule, a converter that yields minimal sonic change between internal and external clock modes, has a good capability to lock well to external clocks (clean clock-locking circuitry). When the sound changes a lot between internal and external clocks, the AD locking circuitry is "less then optimal". Any differences are much more indicative of a poor AD locking circuitry, and much less indicative of a better external clock. If the external clock changes the sound, look for an AD with a better external locking capability, not for a better clock source.

Of course clocking is only one aspect of conversion and too much focus on clocking can take ones attention away from the other important aspects. This is very important to keep in mind! The clock sellers have made the clock be center stage. It is not! Clock technology is very simple relative to conversion technology, and keeping things in perspective is very much in order.

When dealing with that one aspect (clocking) it is possible to make an AD with very good capability to lock cleanly to an external clock. It can be almost as good as internal, and from a practical standpoint even “as good”, but not better than internal.”

So unless the Fairlight Audio Interface is completely broken (which I honestly seriously doubt), using its internal clock will be fine. The addition of the Antelope Clock would not improve the system, in fact it may actually increase jitter.

As you know, the Merging HAPI is PTP Master here, with the DAD AX32 as PTP slave, and Resolve simply locks to the PTP Master. An external clock is completely unnecessary in my system.
[*]MacPro 2x Quad 2.26GHz Xeon | OS X.11.6 (15G20015) | Resolve Studio 15 beta 5 |
14GB RAM | NVIDIA Quadro K620 (346.03.15f14/CUDA 8.0.83) | IntensityPro 4K (10.9.11)
[*]Merging HAPI | Ravenna CoreAudio driver 2.0.33517 | DAD AX32
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JPOwens

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Re: Who is using external clock with DaVinci Resolve? How?

PostFri Feb 23, 2018 8:06 pm

Just to throw some numbers onto the argument. Crudely, although I don't do a lot of audio processing, seems to me there are some key clock rates that do really need to be very stable, like 96 kHz.

Remember that even Standard NTSC has been operating at a target bit-rate and stability much higher than that. UHD multiplies that again:

SDI capable FPGAs incorporate transceiver blocks with internal PLLs used to multiply up a reference clock (e.g. 148.5MHz) to the transceiver’s required bit rate. For 12G-SDI the physical layer bit rate is 11.88 Gbps, or 80x the reference clock.


So, even an internal BlackMagic video card is marching to a drum that beats about 123,000 times (?) for every audio sample. Compared to a normal human resting heartbeat, thats 34 hours between audio samples.

jPo, CSI
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Michael Holmes

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Re: Who is using external clock with DaVinci Resolve? How?

PostFri Feb 23, 2018 10:20 pm

Thanks, Reynaud. We appreciated your messages on this topic.

In all the technical detail, we were missing the basics:
1. Our system is a simple one. All video and audio is imported, there is no recording (i.e., ADC) occurring in post. The hardware is very limited: computer (w/PCIe expander/rack hard drives/tape deck/power supply/etc.), monitor controller/clock (Antelope Eclipse), Barefoot monitors, surround receiver/monitors.
2. Clocking is important when conversion is occurring, and we have no conversion taking place (except for the DAC for monitoring).

So, with a simple system and no conversion taking place, clocking is not an issue. Basically the audio 0s and 1s were set when recorded in the field with the Sound Devices recorder, and we don't alter this in post.

This is a great answer. :)
Thanks to all for the help.
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Andrew Kolakowski

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Re: Who is using external clock with DaVinci Resolve? How?

PostSat Feb 24, 2018 11:25 am

Clock is only important for A/D and D/A stage. While you are in digital world dooing some adjustments clock plays no role.

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