Looking carefully at the results, I can see the instant superiority of the Digital Betacam master, in both the luma Y' component and both the chroma Cb and Cr components. It's hard to make out, but the Y' on the BetacamSP captures is slightly softer than the original Digital Betacam and the DVCAM. Again the DVCAM over SDI is practically identical to the DVCAM over FW after it has been smoothed.

The chroma on the BetacamSP is very interesting as it is significantly softer than the 4:2:2 chroma of the Digital Betacam, but also appears slightly sharper than the 4:1:1 chroma of the DVCAM. The BetacamSP to DVCAM dubs look worst of all, which is nor surprising given that they have gone down an extra generation compared to the other examples.

To see this more clearly, I have produced two sets of differences, the first is a plain difference, the second has the differences scaled by a factor of 8. I could not scale the differences to the factor of 128 as the differences are very much more pronounced.

Differences are unscaled in this diagram.

Differences scaled by a factor of 8.

The scaled difference pictures are very interesting. I'm not quite sure why the Y'CbCr colour difference shows such a large variation between DVCAM Firewire smoothed and DVCAM SDI, but again, the luma components are practically identical and the chroma components very similar. It's hard to say which is better, but I'd give the edge to the SDI chroma upsampling, but it a difference that's unlikely ever to be visible without such analysis. Final Cut Pro's scaling algorithm is fairly poor, so if a better scaling algorithm was employed it is doubtful wether even this slight difference would remain between the two methods.

The BetacamSP shows the larges luma differences from the original Digital Betacam. It is interesting how the right hand edge of the boy's red shirt shows a significant luma difference, probably caused by a subtle but noticible ringing / sharpening of the image coupled with the slight blurring noticed in the previous test. The chroma of the BetacamSP is also showing significant differences from the original Digital Betacam, of similar magnetude to that of the DVCAM differences, but again we see clearly that the chroma resolution is somewhere between that of Digital Betacam and DVCAM.

To allow you to see the subtly lower resolution of the BetacamSP luma compared to the Digital Betacam and DVCAM, I have extracted out just the luma differences and placed them side by side. Again, it's tricky to see but I'm noticing ringing / sharpness artifacts on the right hand edge of the boy's shirt and a thickening in the stalk and head of the grasses on the left of the picture. There is a general softness and subtle lack of detail in the BetacamSP luma.

I found the differences above hard to see so I've enlarged the images in Photoshop by a factor of 4 (nearest neighbour scaling) to allow them to be seen more clearly.

Test 4 - The 10bit difference

During the course of these experiments, it came to my attention that there is some confusion in some people's minds over whether Digital Betacam is a 10bit format or an 8bit format. In digital video, 10bit or 8bit refers to the precision of the quantisation of the luma and chroma components. A 10bit format has four times the precision compared to an 8bit format, so there should be a significant and measurable difference.

This test involved the use of Apple's Shake due to it's floating point compositing precision and flexible tool set. I also recieved some help from Apple Shake support in the creation of some of the analytical scripts.

First, to get an idea of what a know 10bit and know 8bit video looks like under the test script, a 16bit gradient from black to white was created in Shake, and exported as a 8bit movie using the 8bit uncompressed Black Magic Design codec, and also exported as a 10bit movie using the 10bit Black Magic Design codec. These original movies were kept for comparison and also dumped to Digital Betacam tape, and recaptured over SDI using the KonaSD with the 10bit Black Magic Design codec.

By examining the output of the PlotScanline1 node, I hoped to be able to determine 8bit or 10bit by looking at the smoothness of the resulting line.

The resulting image showed to me that Digital Betacam is indeed a 10bit format.

Next was the development of the script to allow me to determine whether an arbitary image was 8bit or 10bit. This involved an adition of a ColorX node and Histogram node to the script. The idea is to take the scanline and colour each pixel in the line a different shade of grey depending on it's height, and then to plot the entire image as a histogram. The resulting graph is scaled so that it displays full scale, and any gaps will show where there is no luma value. Of course, it must first be tested on known data:

From the above images it is clear that 8bit images leave clear gaps in the histogram, whereas 10bit images do not. Next I repeated the experiment upon the Digital Betacam video and dubs to BetacamSP and DVCAM we used for the earlier chroma sampling experiments. Because these are colour videos, the resulting histogram was also displayed in colour, although in RGB rather than Y'CbCr. There is no option in Shake to deal with video in it's native Y'CbCr format and any format conversion in Shake was found to muddy the results somewhat. Shake was chosen for these experiments because of it's ability to handle high precision video, and it's useful tools for analysing images.

From these histograms we can see quite clearly whether a video format is 8bit or 10bit. This does not, however mean that the quality of the video is 8bit or 10bit, just that it's using or not using 8bit or 10bit quantisation levels. For instance the 10bit DVCAM image does not show the all missing quantisation levels that the 8bit formats do - it appears to be somewhere between 8bit and 10bit. We know that DVCAM is 8bit in nature so they filling in of the missing quantisation levels could be caused by the transfer to Digital Betacam or the 10bit codec used, or perhaps that Shake is dealing with the video as RGB rather than Y'CbCr. BetacamSP, being an analogue format, can be digitised at whatever quantisation you choose, whether that be 8bit or 10bit. That does not mean that BetacamSP is equivalent in quality to 10bit, however, as noise from the analogue tape will be a limiting factor.

Conclusion

The limiting factor in the quality of DV in Final Cut Pro is related to the method by which the 4:1:1 chroma is dealt with. By using the SDI output into an uncompressed Final Cut Pro editing system, DV chroma can be quickly and easily upsampled to 4:2:2, although the upsampled chroma is visually lacking compared to a reference 4:2:2 Digital Betacam original. If SDI outputs are not available, then a traditional Firewire approach to DV capture can be used if care is taken over the workflow to ensure that the DV 4:1:1 chroma is upsampled before a chroma key or the DV footage is "bumped" up to uncompressed to be mastered out to Digital Betacam or other 4:2:2 format video tape. There is a quality difference between the two approaches. The luma and chroma components are practically identical in both cases, but the chroma generated from the hardware upsample in the DSR-1800 DVCAM deck is slightly closer to the original Digital Betacam 4:2:2 chroma, but the difference is of such a small magnetude for me to recommend you pick your DV capture method based upon workflow requirement rather than any quality difference between the two methods.

By including BetacamSP in these experiments I hope I have broadened the reader's understanding of how both DVCAM and BetacamSP compare to a reference of Digital Betacam. After performing these experiments and carefully examining the video footage, I find it hard to decide wether BetacamSP is better or worse than DVCAM, because although BetacamSP offers a quality of chroma that appears to be about half way between that of DVCAM and Digital Betacam, it's luma resolution softness and edge enhancement ringing detract more visibly. The chroma differences would seem to require special analysis to easily determine differences by eye, whereas the luma differences are obvious as the video is playing without the aid of analysis tools.

However, this is not the end of the story. I have spent the last year working with the chroma components of DV format video in an attempt to develop smart algorithms that would allow the quality of upsampled chroma to be improved beyond what "simple" interpolation offers especially for use when chroma keying from DV sources or for up-rezzing DV to HD formats.

Again using the footage of the boy in the red shirt, I applied two versions of my algorithm, labled G Nicer (a) and G Nicer (b). One of the benefits of using your own algorithm is that you can tweak the settings to your own liking. The (a) and (b) represent different settings of the same algorithm. The version of G Nicer being used is a beta of V2.0.