Lots of older avchd is constant bitrate, which is not very efficient by any measure.
VBR avchd isn't that great at compression either, but when the headroom is needed for motion detail, the results can be stunning.

Its also why rate factor encoders (x264) can reduce bitrates to less than half in many cases without perceptible loss of quality.

Get the fattest cards that work consistently with your camera, and shoot at 60 p when warranted.

"...and shoot at 60 p when warranted."

Considering my mix of cameras, I will shoot at 60p.

My CX900 can do 30p at a high bit rate but my other cameras can not do so. So keeping all of them the same makes it easier to edit for me.

I would say what the bit rate number are but I cannot remember them right now. My mind is very tired after a period of no power because of some big storms that blew through here.

Without power, life is a bit rate of zero! Same as without love.

VBR is useful for offline encoding, or if the compression have a "ceiling" to spend the saved bits.

as AVCHD are already very close to the limit 25mbps, VBR is not beneficial.

i do my blu-ray and dvd encodings in VBR 2-passes most of the time, only do 1 pass CBR when the max bitrate is already in use (where VBR would not have benefits)

variable bitrate modes are most useful when dealing with a fixed final size file or when you need the most quality for a particular size.

"as AVCHD are already very close to the limit 25mbps, VBR is not beneficial."
25Mbps is a de facto AVERAGE bitrate for High Profile Level 4.x MP4; there is no such limitation imposed on AVCHD peak VBR bitrates, which can and do soar into the 40's.

I believe the maximum peak bitrate for Level 4.2 is about 60Mbps.
Some original AVCHD and all AVCHD 2 is High Profile, Level 4.2 MP4.

That indeed, represents a HUGE advantage for high motion video, leaving CBR 25 in the dust.
None of this makes any difference at all if you are shooting talking head interviews.

the specs says "less then 24mbps".

http://www.avchd-info.org/format/OverviewChart1.jpg

and never seen a AVCHD file even reach to 25mbps.

Don't know where you dug that up, but it "might" be the original 2006 proposal, which was Level 4.0.

The AVCHD 1 spec was indeed maximum AVERAGE bitrate of 24Mbps, and absolute Level 4.0 bitrate around 30Mbps (for 1920x1080).
Even 'way back then, both Panny and Sony shot VBR, I believe.

And I'm 100% certain I already covered that above.
I'm also certain you know the difference between fixed, average, and peak bitrate.

Since 2011, the AVCHD 2 spec is all Level 4.2.
That's 28Mbps average (AVCHD 2 Spec), the absolute Level 4.2 peak being about 60 Mbps.

AVC Levels

Yup, allowable peak bitrates more than doubled with Level 4.2
Thanks for confirming.

50i or 50p AVCHD will have a lower bitrate than 60i or 60p for the same frame quality.

When shooting 1920x1080x50p AVCHD m2ts, my camera (Panasonic HC-X920M) produces typically 23 Mbps average and 25 Mbps maximum video bitrate, using High@L4.2 profile.

The maximum system bitrate for AVCHD 2 is given as 28 Mbps (the max video bitrate will therefore be lower).
https://en.wikipedia.org/wiki/AVCHD
http://www.avchd-info.org/format/index.html

AVCHD recording in real time with high quality and relatively low bitrate requires a tremendous amount of compute. Cameras are designed to "fit" within a certain TDP, thermal design power. Besides limited battery power, there is also a limit to how much heat these devices can dissipate.

One way to increase video quality while keeping down compute is to increase the bitrate and this is the balance that designers have to find. The optic block, encoder, and CCD all have to be matched for optimal performance so that their camera can compete at a certain price point. The top bitrate available is what the engineers most likely feel extracts maximize performance from their camera. The lower bitrates are of course subjective reductions in quality.

I always record at the highest bitrate and resolution that the camera offers because I want to work with the best source possible when editing.

+1

One would expect such an in depth answer from Bruce Banner, not Hulk.

VMP

It appears that malowz and I were both wrong. So there you have it.

Years ago I was doing some writing and I was able to interview a couple of the engineers from the major camera manufacturers. They explained this to me.

If you think about it it really does make sense. You can use Handbrake or Ripbot and transcode a 25Mbps stream down to 8Mbps with very, very good results. But the problem is the compute (even with dedicated hardware) required to achieve this quality in realtime would require a large TDP and corresponding high cost. The interesting question now would be to compare the video processing engines in today's camera's from those of 5 or 6 years ago. How much has the quality of the transcode increased at the same bitrate since surely hardware has become more powerful within the same TDP due to process shrinks and architecture improvements?

Luckily memory/storage is pretty cheap these days and as long as the engineers give us cameras with nice high bitrates we should be good;)

[I]"Luckily memory/storage is pretty cheap these days"[/I]

The memory required for the latest "bleeding edge" cameras is not cheap, pretty much the same as a roll of 35mm film with the same recording time. On the upside you can reuse the memory cards. The issue here is that you need a lot of memory and it needs to be as fast as it can be, at around $2K / card I wouldn't want one accidently going into the wash

As for power consumption; the latest cameras from Sony certainly do use a lot of power. Even though fan cooled they get pretty toasty. I think all the cameras that can record XAVC fall into this category.

Back to the OP's question:

[I]"There is a certain minimum bit rate that a particular resolution needs and I do want the bit rate to be reasonably above that, but I can't help but wonder that those extra high bit rates might be a bit of overkill."[/I]

There's not as strong a correlation between resolution and bitrate as there is between scene complexity and bitrate requirements. Noise from the sensor adds dramatically to scene complexity, by definition noise is not compressible. The artefacts produced from lossy compression of noise are even harder to compress. So even though two recordings of the same scene may appear visually identical out of the camera that may not hold true for the final rendered output.

Bob.

Yup, hardest scenes to encode without artifacts are trees with the leaves shaking in the wind and an ocean surface with lots of wave complexity. I've never shot AVCHD but in MPEG2 it gets blocky real fast unless you use a very high number for the Max bitrate in VBR encoding.

There certainly is a minimum bit rate for the nature of the content. Unfortunately, the delivery method may not be able to support the required minimum.

https://www.vegascreativesoftware.info/us/forum/it-ain-t-gonna-look-good-at-3-mbps--98139/

The most demanding material I've encountered is open water, more than once. The rustling leaves are right up there for gluttonous bitrates as well.
A not-so-distant third goes to Sunday picnic videographers.