I understood that on Consumer camcorders HDR is achieved by using alternate frames for + and - EVs
I know nothing about HDR in Panasonic camcorders. There's no proper documentation of it. If every second frame had a different exposures you would be able to examine this by single stepping through the MP4 frames in VirtualDub. I've done so, and the exposure of every frame in both L & H HDR modes is similar.
From
Panasonic WX970 Camera Is a w850 on Steroids! - Hi Speed Cameras
It is clear that HDR Mode is tying with the S curve and it is delivering a less processed image. It looks overly soft and flat which in post could yield a better image. Panasonic claims that two simultaneous images are being combined to produce it: “world’s first integration to the consumer full-HD camcorder*. By combining two images taken with different exposure, blown highlights and blocked shadows are suppressed even in backlighting.”
Perhaps it is shooting at 100fps internally, combining the two differently-exposed frames on-the-fly, and outputting 50fps. The framing on the V750 in slo-mode is tighter in slo-mo than in 1x, indicating that it's using a smaller portion of the sensor and upsizing to 1080p50. I think this is because the V750's processor can't handle that many lines at 100fps internally.
I've just checked. The framing in slo-mo in the V770 is still smaller than in 1x, but bigger than the V750's, suggesting that more horiz. lines are being used for the upsizing.
I didn't expect this. It could be that a more powerful processor, a faster sensor readout speed, or both, is allowing this. This should make the slo-mo in the V770 a bit sharper than in the V750, which is noticeably softer than 1x, perhaps due to the amount of upsizing.
It may also mean that V770 HDR is shot at 100fps internally, with a smaller frame, then upsized, and may be less sharper than normal mode. I can test this by shooing text at a distance.
There's not a cloud in the sky at the moment (7am), so I may be able to shoot one-half of today's 12pm match in HDR (L). The high sun-angle at this event time on a clear day will make the sun-lit-to-shadowed transition look fairly harsh, so HDR may actually help.
Dan.