A7SIII S&Q Mode Bitrates

[Randy Strome]

Hi guys,

I am loving the A7SIII and particularly the quality of the higher frame rate, all I footage. But there is something that will not register for me. Please help me understand what is going on with the bitrates in S&Q mode.

If I am shooting 120 FPS XAVC S-I using 24 FPS as a base rate, the bitrate is 240 mbps.
If I am shooting 120 FPS XAVC S-I using 60 FPS as a base rate, the bitrate is 600 mbps.

It seems to me that the 120 fps would determine the bitrate as this would be the recorded rate in both instances. It seems that the 24 or 60 rates are essentially just playback rates (similar to a predetermined timeline framerate).

Is that all correct? What am I missing?

Many thanks!

[Aaron Jones Sr.]

When you divide 24fps into 240 you get 100mbs per frame. When you divide 60fps into 600mbs you get 100mbs. So it is Sony's way of keeping the bitrate the same for retaining enough I formation to have the same quality image. I hope this helps, Cheers!

[Randy Strome]

Originally Posted by Aaron Jones Sr.

When you divide 24fps into 240 you get 100mbs per frame. When you divide 60fps into 600mbs you get 100mbs. So it is Sony's way of keeping the bitrate the same for retaining enough I formation to have the same quality image. I hope this helps, Cheers!

Hi Aaron, thanks for your reply. That would make sense to me if we were recording at those frame rates (24 and 60) but in S&Q mode in both cases we are actually recording the full 120 FPS. So I am thinking it would be:

240/120=2
600/120=5

Looking at it another way, when I record at a base rate of 60 FPS which forces "600 Mbps", Catalyst Browse is reporting 570 Mbps whether I am choosing S&Q 120 FPS, 60 FPS or 30 FPS as a recorded frame rate. If i am looking at it correctly (and using the camera numbers):

600/120=5
600/60=10
600/30=20

And it is even reporting the S&Q time lapse options like 1FPS at the full bitrates. For instance I have done some time lapses at 1 FPS (recorded frame rate) with a base frame rate of 24 FPS. The camera is recording 1 FPS but the bitrate is showing as 228 mbps in Catalyst browse. So using the camera numbers again:

240/1=240

That is really confusing to me.

[Christopher Young]

if you want Sony's exact breakdown on the A&Siii S&Q rates it's all explained below. The 1200Mbps included.

Chris Young

[Randy Strome]

Hi Chris,

Thanks for your reply. I am not recognizing those bitrates as anything offered in the camera or in the manuals. Could you check that this chart is from the a7sIII? The CFexpress cards, which are the fastest internal media max out at a write speed of 700 Mbps (read at 800 Mbps) so I am not sure how 1200 would be possible. This is what Sony has in the manual (pasted below). It seems like for my purposes S&Q will just be a way to force a higher bitrate onto certain recorded frame rates than are not available in any other way. For instance it would be more useful for me to be able to select S&Q 120 FPS as the record frame rate at 24 fps project frame rate at 600 Mbps but that is not an option so I need to set the project rate to 60 FPS to get the higher bitrate and then change the framerate in Resolve to make full use of the slow motion effect. Not a big deal but it is very confusing to me why it would be set up this way in camera.

I really appreciate you both putting your eyes on this. Just thinking through it with you is very helpful. FWIW, I am blown away by these big, high bitrate (600 Mbps) files that you can only get by choosing 60 FPS as the base rate and then choosing your actual record rate using S&Q. I had read some early reports that there was negligible added value to these files, but it is actually a huge difference. The files are beautiful and become incredibly malleable in post. That is why I am so interested in fully understanding this.

When File Format is set to [XAVC S-I 4K]
Rec Frame Rate Record Setting Size Movie compression format
60p/50p 600M 4:2:2 10bit/500M 4:2:2 10bit 3840×2160 Intra
30p/25p 300M 4:2:2 10bit/250M 4:2:2 10bit 3840×2160 Intra
24p* 240M 4:2:2 10bit 3840×2160 Intra
*Only when [NTSC/PAL Selector] is set to NTSC.


When File Format is set to [XAVC S-I HD]
Rec Frame Rate Record Setting Size Movie compression format
60p/50p 222M 4:2:2 10bit/185M 4:2:2 10bit 1920×1080 Intra
30p/25p 111M 4:2:2 10bit/93M 4:2:2 10bit 1920×1080 Intra
24p* 89M 4:2:2 10bit 1920×1080 Intra

[Christopher Young]

I guess an example might help Randy. It helped me work out exactly what Sony is doing.

Talking 60Hz HD here. You can record XAVC S-I HD at 240FPS. Divide 240FPS by the normal video rate of 30FPS.

240/30 = 8. What is actually happening is you are pushing through 8 x as much data as you would be at 30p. What Sony is saying is that to do that you must use a card that can handle a write speed of 8 x the 30p data rate. They are saying 890Mbps.

Likewise in PAL, you can record XAVC S-I HD at 200FPS. Again divide that by the standard PAL frame rate of 25FPS and again you come back to a data rate of 8 x over the standard frame rate of 25FPS.

When it comes to UHD (3840 x 2160) you have slower choices of 100p or 120p in XAVC S-I. These are 2 x speed increases over 50/60p or a 4 x speed increases over 25/30p.

In general terms in UHD, this means that the normal I-Frame bit rate of 500/600Mbps when running at 100/120p is doubled. In NTSC terms that 60p @ 600Mbps, when doubled, is a data rate throughput rate of 1200Mbps.The nominal data rate hasn't changed. it's still 600Mbps (per second). It's that you are pushing it through at twice the rate, 1200Mbps. Hence the requirement for much faster cards.

Back to the HD example. If we now take HD @ 200Mbps at 50/60p and 200/240p at 25/30p we have data throughput rates of 4 x for the 50/60p and as previously mentioned 8 x for 200/240 at 25/30p.

In other words, you are going to need cards capable of running 8 x throughput when recording 200/240FPS in XAVC S-I HD.

Check the bottom of page #21 and onward in the A7Siii manual for the S&Q data rate breakdowns:

https://alphashooters.b-cdn.net/wp-c...iii-manual.pdf

Check the following video for a probably much clearer explanation than my hash of it. Mainly from 6:21 onwards as he discusses S&Q specifically:

Chris Young

https://www.proav.co.uk/videos/sony-...ok-data-rates/

Sony A7s III - An In-Depth Look at the Data Rates - YouTube

[Randy Strome]

Hi Chris,

I am wondering he had the camera when they did that video. What he says at 7:17 seems not quite right. This is the confusing part. He is saying that the best way to get the highest quality 25 for 4x slow motion is by recording 100 Mbps at 25 base rate which gives you 250 Mbps files. That is not really correct. You can get 500 mbps 25 4x slow motion by filming 100 at 50 FPS. You just have to change the rate in post. That way you can get 500 Mbps at 25 fps playback speed. It seems like this would and should be an option in camera since the 50 FPS is really just a playback setting which can be changed later.

I guess i am not understanding the 1200 and 890 Mbps rates or how the type A cards could manage almost twice their read and write speeds. I see that the Type B cards could manage those speeds but Sony is saying that those cannot be used in the A7sIII The maximum actual data rate I am seeing on any files is 570 Mbps and that is the same rate for S&Q record rates of 24, 30, 60 or 120 as long as 60 fps is selected as the base rate.

[Christopher Young]

Obviously still some confusion reigns. Regardless of what is stated in the video let's bring it down to the nuts and bolts of it. It all boils down to the following:

What speed cards do I need to use for different modes?

For general recording in 4K up to 120p, you will have to use at minimum V60-rated SD cards.

Slow-motion with the XAVC S-I 4K setting at 60p will demand a V90 SD card or better.

S&Q at 120p XAVC S-I 4K, which is double speed will require a CFexpress Type-A.

Why? Because S&Q at 120p XAVC S-I 4K is twice your nominal data rate of 600Mbps at 60p. Twice the data rate is therefore 1200Mbps. You are now recording two seconds of information in one second. The only cards that will handle 1200Mbps which represents two frames of data are the new CF Express A designated cards.

You have to look at it from the point of view of Mbps. If UHD 60p is normally 600Mbps for that one second and you now want to record 120p, 120 frames in a second you will need to record 1200Mbps.

Whether or not there are better options in post is not the point of discussion here. All I'm trying to do is explain why Sony is stipulating certain cards for certain data rates.

Trust this helps.

Chris Young

[Randy Strome]

Hi Chris,

Thanks again.

Sony's CFExpress Type A cards max out at 700 Mbps write speed. I have a number of those. That is the fastest internal media that I am aware of so I am unsure how 1200 would be possible.

When I record the highest bitrate option that the camera allows (60 FPS recorded at S&Q 120 FPS) the camera records at 570 mbps to these CFExpress cards. The camera does the same for all S&Q record framerates so long as 60 FPS is chosen as the base rate.

Here are 3 examples from Catalyst browse. This is the same shot at 120, 60 and 30 S&Q record rate all at the same 60 FPS base rate.

[Christopher Young]

Randy.

Sorry, I think we have confusion here between MB/s and Mbps. Two very different scales of measurements.

If you take one of the cards below as an example that card is rated at 700MB/s for the write speed. That's Megabytes, not Megabits. A 700MB/s (Megabytes per second) card can handle 5600 Megabits (Megabits per second.)

There are 8 Megabits in a Megabyte.

Megabits per second are often shortened to Mb Mbits p/s or Mbps.

So in our case of requiring a throughput of 1200Megabits (Mbps) let's divide 1200/8. 1200Mbps divided by 8 is equal to 150Megabytes per second (MB/s.)

Therefore any card that is capable of better than 150MB/s should in theory be suitable. In practice, this is not the case. What we need for video is a 'sustained' continuous high speed write capability for a non-stop video stream. That's how the designations of V30, V60, and V90 came about in the SD card format.

Given this information, we can see that a v90 card is 'guaranteed' to be capable of recording continuously at a sustained 90Megabytes per second. 90Megabytes per second is not fast enough for the A7Siii when running throughput at 1200Mbps as that is equivalent to 150Megabytes per second.

So we need a card that has a much higher guaranteed continuous write speed than 150megabytes per second. Therefore a card with a write speed of 700MB/s which equates to 5600Mbps is just what the doctor ordered. Dr. Sony in other words:)

Maybe this helps clear up any confusion if that confusion is based around a misunderstanding of MB/s vs Mbps. Frequently done believe me!

BTW the new Sony 'B' series CF Express are now claiming a write speed of 1480MB/s

Chris Young

[Randy Strome]

Hi Chris,

Thanks for pointing that out! I hadn't caught that these cards had so much headroom. That is great. Question, though. Any thoughts on why Sony is only using 570 Mbps (listed as 600) for the largest in camera files. The 3 clips that I posted data for in my post above are XAVC S-I, 60 FPS, S&Q mode which I believe is the highest bitrate setting. I am not seeing any way to utilize the 1200 that you had shown me. If 1200 is available I would love to use it. Possibly I am missing a setting?

[Christopher Young]

An understanding of how Sony's XAVC codec works may help clarify the situation.

Firstly Sony's XAVC S-I codec is a VARIABLE BIT-RATE codec.

This codec will encode each frame using the optimum compression calculated by Sony's proprietary "Pre-coder" to deliver the best PQ quality vs file size. The first graphic shows you the breakdown of a XAVC S-I 4K 50 FPS 10-bit H.264 file. You will notice this particular clip's average bit-rate is 478Mbps. It's from a pretty static scene. In terms of 60Hz the maximum frame bit-rate is 600Mbps. In PAL terms it's 500Mbps. Because this codec is highly efficient it will only use the optimum Mbps to encode a frame. Using any higher bit-rate is inefficient as it will not deliver any better quality. See image #1 for a breakdown of the XAVC S-I codec.

I mentioned "Pre-encoder".
Sony's implementation of H.264 is the ONLY one that uses two-pass encoding. This is possible because Sony is the only company in the video world that has the processing power/speed to "two-pass" an H.264 video frame with high bit-rates in real-time. Other camera manufacturers who are also using H.264 for their I-Frame codec do not have this proprietary pre-encoding capability. One manufacturer who uses a similar I-Frame technology is using 810Mbps to achieve a similar picture quality when it comes to temporal and spatial quality. See image #2 for the Pre-coding processing path. Evidence of Sony's high-speed processing capability is seen at 100p where the A7Siii is delivering an amazing rolling shutter figure of just 8.7ms. A truly amazing number. For comparison Canon's FF R5 in full-frame readout is sitting at 15.5 ms. Admittedly with a higher resolution sensor.

If you have the time and inclination to learn more about the XAVC family. Which any cameraman and editor should do if they are keen to learn how to get the best out of their kit and editing processes watch the video below. Pay attention to the Pre-coding dual-pass comments at 11:20

Finally. No, you are NOT missing a setting. YOU ARE USING 1200Mbps if you are shooting XAVC S-I in UHD with S&Q at 120p. You are recording 2 x 60p in one second. One second of UHD at 60p is 'up to' 600Mbps. Recording two seconds of footage in one second is 120p. This is where the "up to" 1200Mbps comes in. It is not a selectable setting on the camera. There is no "1200Mbps" setting. It automatically uses up to 1200Mbps when shooting S&Q 120p in UHD using the XAVC S-I codec.

As I stated the camera because it's using XAVC as a codec will encode very efficiently and use what it calculates on the two-pass pre-encode as what it calculates to be the best and most efficient compression ration based on quality vs size per frame. If you shoot a fairground carousel full of fast motion and then shoot a plain white wall with no movement at all and compare the two XAVC S-I bit-rates you will see a difference in the Mbps used.

Lastly, I've included a table of the "Operating points of XAVC 4K Intra profile for the XAVC MXF File Format (Intra VBR)".

The A7Siii does not use the MXF wrapper but the underlying structure of the H.264 bit-rates are the same. You will note that the Sony A7Siii is using Class 300 XAVC specs which is up to 600Mbps. Class 480 XAVX if it were available on this camera could use up to 960Mpbs per frame at 60p. At 120p (frames per second) it could use up to 1920Mbps. But that's another story. Just remember the highest quality image is achieved when the best bits per pixel depth ratio is encoded. Any bit-rate above that "sweet-spot" is wasted as there is no quality improvement. It only increases file size which is wasteful as it requires faster data paths and larger and faster storage.

More clarity I hope:)

Chris Young

XAVC: Advanced Video Coding Technology - YouTube

[Randy Strome]

Originally Posted by Christopher Young

Finally. No, you are NOT missing a setting. YOU ARE USING 1200Mbps if you are shooting XAVC S-I in UHD with S&Q at 120p. You are recording 2 x 60p in one second. One second of UHD at 60p is 'up to' 600Mbps. Recording two seconds of footage in one second is 120p. This is where the "up to" 1200Mbps comes in. It is not a selectable setting on the camera. There is no "1200Mbps" setting. It automatically uses up to 1200Mbps when shooting S&Q 120p in UHD using the XAVC S-I codec.

Hi Chris,

Absolutely amazing detail. Thanks so much for taking the time and posting that!

That makes perfect sense. You may have forced a lightbulb moment :). I think that I was expecting Catalyst browse to show an increase in bitrate as the framerate increased. It really threw me that in S&Q 30p, 60p and 120p all were reporting 570 Mbps. *BUT* based on your info I just shot 3 timed test clips at 20 seconds each. All S&Q with a base rate of 60p. While Catalyst still reports them all at 570 Mbps (actually 573) the file sizes are doubled from 30 to 60 and again for 120. So, obviously the higher bitrates are happening even though Catalyst is showing the same rate.

I then shot the same 20 second clips without S&Q in 60p and 30p (there is no option for 120 without S&P with all I). The 60p clip is again at 570mbps but the 30p clip is shows 287 Mbps. The file sizes however both match the S&Q for the same framerates.

Do you know what is going on there? Is it common to report S&Q bitrates based on playback frame rate? If so, I suppose that would explain it.

Either way, your pointing out that 1200 Mbps spec (I had entirely missed that and had not heard mention of it anywhere except from you - many thanks) and clearing up the capabilities of these Type A cards (I had missed that as well and thought we were just above the 600 Mbps threshold) has mostly set me straight.

Again, Many thanks for your time.

[Christopher Young]

Quoting you:

"*BUT* based on your info I just shot 3 timed test clips at 20 seconds each. All S&Q with a base rate of 60p. While Catalyst still reports them all at 570 Mbps (actually 573) the file sizes are doubled from 30 to 60 and again for 120. So, obviously, the higher bitrates are happening even though Catalyst is showing the same rate."

You are correct. Catalyst and most other analysis software will only report the bit-rate in Mbps per second for the "base framerate". In your case it was 60p. If you double or triple that framerate then you will double or triple the bit-rate. In your example, the nominal bit-rate of 600 (573 in your case) would double at 120p. That would in fact be 1146Mbps that you are pushing through.

Yes, it can be a bit confusing. I think you've got it now though. Happy shooting!

Chris Young