In my opinion, as was the point of this post, the best option is to acquire as DV25, via Firewire. The original DV material can be moved to and from DV tape via firewire infinitely without any further loss of quality – generationless.

The point is that compression, if you’re shooting on a DV25 format, is applied at the time of acquisition. What is stored on the tape is a 25Mbit/s stream of compressed video data.

In ‘capturing’ the DV25 footage into an Avid DV25 resolution via Firewire you are transferring that video essence (the 25Mbit/s compressed video data) as a data stream onto the computer, where the data is wrapped in an MXF or OMFI wrapper. It is lossless – the data is transferred from the tape (or disc, or drive) without change. No further compression or recompression occurs.

Whereas, if you were to capture the same DV25 footage using a baseband connection (SDI or Analogue Component) then that video would be decompressed from the player, and then recompressed (the same 25Mbit/s Intraframe DCT compression) by the Avid software. For a DCT compression this can be quite damaging, especially if any translational shifts occur, in which case the DCT blocks (between acquisition compression, and subsequent NLE compression) become misaligned will significantly degrade the image.

If capturing via Firewire in Avid, from a DV25 source, then a matched DV25 resolution is fine, and infact lossless (if possible all subsequent rendering should be at a greater resolution, such as 1:1). However if the capture is taking place via SDI or Analogue, then it’s vital that a greater resolution be used (1:1 ideally).

However if you capture DV 25 footage

…or you can do a Google search for DVCAM format overview to read SONY’s white paper document on the DVCAM format. Once you have the document, see the Video Signal Processing section, page 6. It describes the sampling, blocking shuffling, etc. processes of how the bits are stored on the tape.

It doesn’t matter how the bits of data are recorded to the format, the compression process is applied during the ‘Sampling’ (“The sampled video data is reduced by a factor of 5:1 using bit rate reduction…”) and ‘Shuffling’ (“In the DVCAM/DV formats, the compression process is applied over five macro blocks gathered from five different super blocks.”) phases of the signal processing, hence the 5:1 compression.

However if you take that same tape and capture it by SDI then you’re capturing a baseband video stream and recompressing it. The worstcase scenario basically being capturing DV25 footage into a DV25 Avid resolution using any method other than Firewire. In that scenario the DV25 compressed footage is being unpacked into a baseband video stream, sampled by the Avid and recompressed back to DV25. This will create significant loss that will become visible generational loss quite quickly.

If the source footage is DV25, and is captured (transferred really) via Firewire into DV25 Avid media, edited and output again via Firewire to a DV25 deck there is absolutely no generation loss. The data written to the tape is exactly, bit for bit, the same as what was ‘captured’. However as soon as the footage as any translational effect (scaling, resizing) applied, or has to undergo and pixel manipulation (colour correct, transitions) then new media is created and will be compressed with a DV compression at some point (assuming it’s going back to a DV tape) – at which point change has occurred and generational compression becomes an issue.

Generation loss (however slight) occurs when you output from the AVID to DVCAM via any conduit. DVCAM has a native compression ratio of 5:1. When recording to DVCAM the sampled video data is reduced by a factor of 5:1 using bit rate reduction. This loss has nothing to do with AVID output, rather the media you choose to output to.