I will try to write something about creating the archive for the compliance recording purposes from the SD-SDI source. This post is in a response from an repeated inquiries about system that would create such an archive.
Compliance recording in general
Just to remind us – compliance recording serves only one purpose – to prove or disprove that something went on the air at some time. That is the first and the last thing that this technology is used for. There is no any requirement on the content of the signal – it just have to be good enough for someone to see basic stuff that’s in there .
This one business requirement is countered from the other side with need to create the archiving system to be as affordable as possible. And the cost of the system, if we count out the software involved (system and application part) is highly dependent on the:
- picture quality
- number of channels recorded
- number of possible outputs needed
- and days of the archive that should be kept
Picture (ie. video) quality is directly proportional to the BITRATE of the video that is recorded. More bitrate, more apparent quality, both in picture resolution, object motion, and so on. For example, most recorded videos that you might have on your computer are in between 700 and 1000 kbit and are recorded with DivX or similar encoder. Watching a movie at 1000 kbit bitrate is highly enjoyable, and everything above that is required ONLY for HD or HD-Ready content.
Compliance recording means recording what’s on the air
Many people forget that when you do compliance recording, you have to record what was coming out of your transmitter array, not the final that you are broadcasting. In case your link went down, or your output HF amplifier burned, your endpoint picture will be NOTHING, and if you are recording your output, there will be a great discrepancy in your logs.
So suggestion: drop the thinking about recording SD-SDI, and try to record what comes back from the air, in the form of the analog or DVB-T signal. That is the REAL broadcast that your viewers see.
The conflict of interest
We all want stuff to be as cheap as possible. In order to build a recording system that is affordable, we have to balance between various things, but here I’ll try to explore the differences when SD-SDI signal capture is required.
I will postulate this input parameters for the archive that will serve as example here:
- video bitrate: 512 kbit
- audio bitrate: 64 kbit
- archive duration (number of days to keep from today): 92 (3 months)
- number of channels that needs recording: 4
- required outputs: both WMV (Windows Media Video) and h.264 (at the same time)
- recording resolution: 3/4 of PAL D1 picture size so: 540×432
In order to calculate hard drive space requirements for this here, I’ll use the online bitrate calculator here on this blog.
Table below taken from the calculator itself.
So we need 2200GB of net drive space. In order to provide that kind of space with some fault tolerance, I would recommend having two drives such as WD CAVIAR GREEN 2500GB connected in a MIRROR VOLUME (RAID1). There is no need for additional system drive, and almost all new motherboards can provide on-board implementation of RAID1 – mirroring.
I’m kind of beating around the bush here – because main point of this article should be to provide you with a choice of whether to use original SD-SDI signal or convert to composite signal (or use the signal that comes back).
Osprey vs DeckLink
The choice that lies before us is either to use Osprey 460e card, or a DeckLink Quad card. List price for the Osprey is about $1200, and two DeckLink Quad card is about $1000. So, the first impression is that you’ll go cheaper with DeckLink. But…
So far, the Osprey has proven itself to be absolute master of video capture. Every system deployed so far didn’t have any problems with the card whatsoever. You plug it in, install drivers, and it works. And with Osprey 460e, you’ll use only ONE PCI-e slot. Other slot, if there is any, will be used by the graphics card.
Same thing about the slots applies to DeckLink as well. However, there is “now shipping” image below the DeckLink Quad card name, and it implies something – it is red-hot new. Despite the fact that VideoPhill Recorder will work with it, I don’t know how reliable it will be in 4 channel simultaneous recording situations.
My point – it has to prove itself – and if it does, it will be a great addition to VideoPhill arsenal.