Hi. I actually contact David Heath regarding this thread. I don't know if he has a user account but he has written to me and has given me permission to quote his emails.
Here they are:
email 1
"As far as the DVXuser posts in that thread go, then a couple of points. The whole 4:x:x notation should really be only used in relation to a RECORDING system, and it starts to get very messy when it gets used more widely. The important essence is that it denotes the ratio of recorded colour samples to luminance samples, just as the article that David Jimerson posted to in that thread says. What’s important in this context is what happens in the vertical direction, and that thread is assuming 1080 video, so we’ll consider a vertical line of 1080 pixels.
In this case, 4:2:2 implies that 1080 luminance and 1080 chrominance samples will be recorded vertically, 4:2:0 implies that 1080 luminance and 540 chrominance samples will be recorded. The numbers say NOTHING about what the front end of the camera is generating. The assumption in the DVXuser article is that the head output has equal luminance/chrominance resolution, and for a non-pixel shift 3 CCD camera that’s true. But cameras with Bayer sensors, or those using pixel shift, have a luminance resolution that is inherently greater than the chrominance resolution. I stress there is nothing wrong with that. It matches the human eye, which is inherently more sensitive to changes in brightness than in hue, and is the whole reason why colour subsampling is used in broadcasting.
In the case of the 200/171/151, there are 540 r,g,b pixels each vertically, so at first sight the luminance and chrominance resolutions may be thought to be 540 (lens permitting). In practice, the r and b pixels are offset from the g to enhance the luminance resolution (by a factor of about 1.5X overall, and because the camera uses 2 dimensional shift, it’s about sq root 1.5 on each axis, say 1.25x) but this doesn’t do anything for the chrominance resolution. That 1.5x figure comes straight from the Panasonic Technical White Paper, incidentally, ( ftp://ftp.panasonic.com/pub/Panasonic/Drivers/PBTS/papers/AG-HVX200.CCD-WP.pdf ) at page 3: “Spatial Offset technology will improve the resolution by a factor of 1.5 provided the gain in resolution is not offset by the quality of the lens.”
Hence although DVCProHD is capable of RECORDING 1080 chroma samples vertically (it’s a 4:2:2 recording), the camera front end can only supply it with information corresponding to 540. This was my analogy to a container half empty. (If anyone disputes that, ask them what happens when confronted by a blue lines on black test chart, such that only the blue pixels produce an output. It CAN’T resolve better than 540 vertically.) Now take a camera such as the EX which has 1080 r,g,b pixels vertically, and here the front end IS capable of supplying 1080 chroma samples. Unfortunately, the 4:2:0 subsampling throws every other one away, so again the nett result is 540 RECORDED chroma samples. Hence the analogy of trying to pour a litre into a 0.5 litre pot, half gets spilt. Far from “rationalizing why, in theory, the EX1 should still be better at color than the 200 or 170” as David Jimmerson suspects, I’m actually saying that they both have more or less the SAME vertical chroma resolution RECORDED, 540! (Though the EX is capable of supplying better via HD-SDI.)
Now move on to the 151, and here the 4:2:0 recording of AVCCAM means it has the capacity to record 540 vertical chroma pixels – which matches nicely what the front end is capable of. Hence all three examples end up with effectively 540 vertical res recorded.
It’s a different topic, but it’s worth thinking about recording 720p instead of 1080. The 151 720 codec records full raster 1280x720 in this mode, DVCProHD records 960x720. The resolution of the front end should be around (960x1.25)x(540x1.25) from theory (about 1200x650), which matches an AVCCAM 720 raster very well. Record to 720p DVCProHD and quite a lot of horizontal detail is lost. Hence for a 151, 720 recording should preserve as much luminance detail as 1080, whilst giving the compressor a much easier time.
What isn’t clear from the original quote at the head of the thread is that in the original DVinfo thread it came from I was replying to a question about how effective it would be to use a CV XDR with a 151 (see post 62 of the original thread), and was addressing a very specific circumstance rather than a general 422/420 one. Take that into consideration, and what was quoted makes more sense.
At the end of the day, all this is only one factor in making a camera choice, though it’s good to understand the technicalities. Personally, I think the SDHC card ability of the HMC151 versus P2 of the 171 is a far, far more significant factor than nit-picking over resolution numbers, and the extra difficulty of editing AVCHD can be overcome by transcoding on import. If you’re looking for a “B” camera to an otherwise 2/3” P2 shoot, the 171 and P2 may be the way to go, otherwise the advantages of SDHC seem to make the 151 a better choice than a 171 for the vast majority of people.
Hope that helps answer your query,
David"
Here they are:
email 1
"As far as the DVXuser posts in that thread go, then a couple of points. The whole 4:x:x notation should really be only used in relation to a RECORDING system, and it starts to get very messy when it gets used more widely. The important essence is that it denotes the ratio of recorded colour samples to luminance samples, just as the article that David Jimerson posted to in that thread says. What’s important in this context is what happens in the vertical direction, and that thread is assuming 1080 video, so we’ll consider a vertical line of 1080 pixels.
In this case, 4:2:2 implies that 1080 luminance and 1080 chrominance samples will be recorded vertically, 4:2:0 implies that 1080 luminance and 540 chrominance samples will be recorded. The numbers say NOTHING about what the front end of the camera is generating. The assumption in the DVXuser article is that the head output has equal luminance/chrominance resolution, and for a non-pixel shift 3 CCD camera that’s true. But cameras with Bayer sensors, or those using pixel shift, have a luminance resolution that is inherently greater than the chrominance resolution. I stress there is nothing wrong with that. It matches the human eye, which is inherently more sensitive to changes in brightness than in hue, and is the whole reason why colour subsampling is used in broadcasting.
In the case of the 200/171/151, there are 540 r,g,b pixels each vertically, so at first sight the luminance and chrominance resolutions may be thought to be 540 (lens permitting). In practice, the r and b pixels are offset from the g to enhance the luminance resolution (by a factor of about 1.5X overall, and because the camera uses 2 dimensional shift, it’s about sq root 1.5 on each axis, say 1.25x) but this doesn’t do anything for the chrominance resolution. That 1.5x figure comes straight from the Panasonic Technical White Paper, incidentally, ( ftp://ftp.panasonic.com/pub/Panasonic/Drivers/PBTS/papers/AG-HVX200.CCD-WP.pdf ) at page 3: “Spatial Offset technology will improve the resolution by a factor of 1.5 provided the gain in resolution is not offset by the quality of the lens.”
Hence although DVCProHD is capable of RECORDING 1080 chroma samples vertically (it’s a 4:2:2 recording), the camera front end can only supply it with information corresponding to 540. This was my analogy to a container half empty. (If anyone disputes that, ask them what happens when confronted by a blue lines on black test chart, such that only the blue pixels produce an output. It CAN’T resolve better than 540 vertically.) Now take a camera such as the EX which has 1080 r,g,b pixels vertically, and here the front end IS capable of supplying 1080 chroma samples. Unfortunately, the 4:2:0 subsampling throws every other one away, so again the nett result is 540 RECORDED chroma samples. Hence the analogy of trying to pour a litre into a 0.5 litre pot, half gets spilt. Far from “rationalizing why, in theory, the EX1 should still be better at color than the 200 or 170” as David Jimmerson suspects, I’m actually saying that they both have more or less the SAME vertical chroma resolution RECORDED, 540! (Though the EX is capable of supplying better via HD-SDI.)
Now move on to the 151, and here the 4:2:0 recording of AVCCAM means it has the capacity to record 540 vertical chroma pixels – which matches nicely what the front end is capable of. Hence all three examples end up with effectively 540 vertical res recorded.
It’s a different topic, but it’s worth thinking about recording 720p instead of 1080. The 151 720 codec records full raster 1280x720 in this mode, DVCProHD records 960x720. The resolution of the front end should be around (960x1.25)x(540x1.25) from theory (about 1200x650), which matches an AVCCAM 720 raster very well. Record to 720p DVCProHD and quite a lot of horizontal detail is lost. Hence for a 151, 720 recording should preserve as much luminance detail as 1080, whilst giving the compressor a much easier time.
What isn’t clear from the original quote at the head of the thread is that in the original DVinfo thread it came from I was replying to a question about how effective it would be to use a CV XDR with a 151 (see post 62 of the original thread), and was addressing a very specific circumstance rather than a general 422/420 one. Take that into consideration, and what was quoted makes more sense.
At the end of the day, all this is only one factor in making a camera choice, though it’s good to understand the technicalities. Personally, I think the SDHC card ability of the HMC151 versus P2 of the 171 is a far, far more significant factor than nit-picking over resolution numbers, and the extra difficulty of editing AVCHD can be overcome by transcoding on import. If you’re looking for a “B” camera to an otherwise 2/3” P2 shoot, the 171 and P2 may be the way to go, otherwise the advantages of SDHC seem to make the 151 a better choice than a 171 for the vast majority of people.
Hope that helps answer your query,
David"
