It should be pointed out that Panasonic has gotten almost everything right with the GH3. They have addressed more user suggestions in one version update than most companies do in three. Not only have they given higher bitrate recording modes, but they also seemed to have increased dynamic range (even if only slightly). The other big feature that wasn't given as much attention is the ability to record the uncompressed signal from the HDMI. There have been rumblings about the color space coming from the HDMI, and it had been noted in many places that the output was only 8-bit 4:2:0. Since the camera isn't final, and not all Panasonic employees were familiar with the finer details, I had initially been hesitant about posting the information. Now EOSHD has an interview with a senior engineer at Panasonic about the issue.
Ah, the format after processing for HDMI – has no compression. It is 4-2-0...This depends on the system internal structure. This one is optimised for the still picture quality also.
When asked if there was any way to bring back this functionality (ideally through a firmware upgrade), he responded by saying:
I cannot at this time because of the structure.
This is what I had been hearing from different sources for a few days now but this is the clearest information I have found regarding the issue. This is a problem for many who were hoping to use external recorders and get high quality 4:2:2 video in ProRes or DNxHD -- or even uncompressed for that matter. The users who would benefit the most from uncompressed output are those who want to do any chroma keying or those who would like to do heavy color grading. While a good chroma key is much more difficult with less color information, color grading is possible (even if not ideal) with 8-bit 4:2:0 footage, as we know from most DSLR footage.
I'm not sure I understand the reasoning for why the HDMI output has to be 4:2:0, and that answer could definitely be lost in translation somewhere. He seems to be saying that the still image quality affects the color space of the HDMI in some way, and this has something to do with the imaging pipeline and the internal electronics. I know a bit about the inner workings of digital cameras, but if someone understands more than me why optimizing still image quality would affect the color space of the HDMI output, I'm all ears. The only possibility I can think of is that it has something to do wit the camera's ability to take a still image without interrupting video recording.
Regardless, the internal recording on this camera is as good, if not better, than any DSLR currently out there. Since the bitrates are already so high and they are also including an intraframe codec, there would be a point of diminishing returns by recording externally anyway. While you can definitely still record externally if you'd like to have the least compression possible, the files that an external recorder would produce will likely not be much better than the internal recordings. It's not likely that the HDMI capability is something Panasonic addresses hardware-wise before the camera is released in November, but I guess you never know, stranger and more unexpected things have happened.