Video Transcript
Advanced Digital Video (Part 4)
7m 4s
00:07
Welcome back! Let’s wrap up with video codecs.
00:11
We found that there are a lot of codecs out there that can deliver nice looking video
00:16
but in doing so they're using up loads of unneeded bandwidth.
00:20
That’s why we created our own codec.
00:22
Q-SYS Shift is true to its name, in that it changes based off the content that’s being transported.
00:28
For example, content with relatively little motion or changes occurring can be transmitted with very little bandwidth.
00:35
However, when full motion video begins the shift codec can adjust and increase the bandwidth as needed
00:42
to send full motion video without any compression artifacts.
00:46
We started with a DCT (or Discrete Cosine Transform) as our base compression algorithm,
00:52
which is also used by the most popular codecs out there today,
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along with both temporal and spacial compression to give you a codec that can adjust to any situation.
01:03
Q-SYS Shift also has a few other features like RFI and RTSP. Let's take a look at all of these in detail.
01:10
DCT is less intense with computations
01:14
and widely accepted when compared to DWT-based codecs which use discrete wavelet transport.
01:21
Spatial compression reduces the amount of data within a frame.
01:25
Think of it like a JPEG. You can take a RAW image from a camera
01:30
and the algorithm will reduce it without any negligible compression artifacts.
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We do this with every frame in video.
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We know that video is really just a sequence of still frames played at a rate
01:42
so our eyes cannot detect the change in frames.
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The change between frames is where the temporal compression is used.
01:50
For example, the image below shows a few frames of a boomerang flying through the air.
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Using temporal compression, we only send the data that is changing between frames.
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Note that the background does not change so that data does not need to be resent.
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Only the data about the boomerang.
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What we get as a result of these two compression techniques is a native Q-SYS device
02:14
well suited for the conference room environment,
02:16
although the NV series is capable of being in any environment.
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For example, a user has a slide deck or a spreadsheet which doesn’t change very often,
02:27
you can see significantly less bandwidth usage in that kind of content.
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Compared to a JPEG 2000 codec, Q-SYS Shift has far less network bandwidth usage with static content.
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However, Q-SYS Shift can easily be deployed where full motion video is required,
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the codec will just shift or adjust based on the incoming content
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and use the right amount of bandwidth that is required for full motion video.
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RFI stands for Reference Frame Interval.
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RFI can be a useful setting within Q-SYS that can be used to help with network recovery
03:00
with increased bandwidth or reduce bandwidth with a reduced network recovery.
03:05
In general the codec uses I-Frames, B-frames and P-Frames.
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The Shift codec doesn’t exactly use these but the idea is the same.
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The I-frame contains the full frame containing information for the full image.
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The P-frame only encodes the changes between the current frame and the previous frame.
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Like the P-Frame, the B-frame encodes the changes between the current frame and the previous frame,
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but also encodes the changes between the current frame and the future frame.
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So, how does this apply to RFI?
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The RFI decides more or less how frequently we send a full frame of data.
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If there are network errors, an I-frame will clean those up.
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However, I-frames require more bandwidth than a P-frame or a B-frame.
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So, how frequently should an I-frame be inserted? The lower the RFI, the more I-frames get inserted,
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which means you get better error recovery at the expense of increased bandwidth.
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The higher the RFI, the worse error recovery but and a decrease in bandwidth usage.
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What does that look like? Well, here's a real-world test using a computer desktop, a static image, at 4K.
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With an RFI of 200 we see a bandwidth usage of around 7 megabits per second.
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That same content with an RFI of 30 will yield a bandwidth of approximately 40 Mbps.
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If you open the NV Series component in Q-SYS Designer Software,
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you’ll will find the RFI setting, with a range between 5 and 255 and a default setting of 30.
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You will also note that you can set the bitrate here.
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This is the maximum allowable bitrate that the algorithm will try and use.
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This is equally applied across all HDMI inputs.
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The NV Series is capable of encoding either a single 4K source or 3 simultaneous 1080p sources.
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If you set the encoder to a bitrate of 600 Mbps and you set it to 4K60 max mode
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then you can transmit a single 4K stream with a maximum bitrate of 600 Mbps.
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Now, if the encoder is set to 1080p60 max mode
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then you are able to share that 600 Mbps across all three inputs.
05:27
300 Mbps for 2 streams, 200 Mbsp for 3 streams. Math!
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One last thing. If an encoder in 4K60 mode attempts to encode more than one 4K60 stream
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then a hot plug event will be triggered and this will change all input EDIDs to 1080p,
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thus allowing multiple streams at a resolution of 1080p or less.
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So, if a 4K max mode is required
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it is highly recommended that the encoder only wire a single AV stream to any decoder,
06:01
try and not wire multiple 4K sources on a single encoder.
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If an encoder is in the 1080p60 max mode then there are no worries for a hot plug event,
06:11
you can stream all 3 inputs simultaneously without worry.
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Real time streaming protocol or (RTSP) is another useful tool in establishing
06:20
and controlling the media during a streaming session.
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The decoder requests the media from the source device and the source will send data as unicast or multicast.
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Also known as RTSP pull streaming.
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This means that video data is not broadcast over the network for no reason.
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Encoders do not send video into the network until the decoder requests the video stream.
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If all decoders are set to a graphic image then no video data is sent onto the network.
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And that’s it for Digital Video. Phew, you made it! Congratulations!
06:57
Thanks for watching and we’ll see you next time.
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Advanced Digital Video (Part 4)
7m 4s