Archived from groups: uk.rec.video.digital,rec.video.desktop (
More info?)
"Gene E. Bloch" <spamfree@nobody.invalid> wrote in message
news:mn.53a47d58d22c5763.1980@nobody.invalid...
> On 8/10/2005, Tony Morgan managed to type:
>> In message <dddgg6$g27$1@nwrdmz01.dmz.ncs.ea.ibs-infra.bt.com>, Chas Gill
>> <Chas.Gill@gollum.btinternet.com> writes
>> Snipped...
>>
>>>> I would suggest that you are mistaken in this instance. The CCD(s)
>>>> output
>>>> goes to the DSP with one configuration, then feeds it to drivers for
>>>> the
>>>> tape recording.
>>>>
>>>> On replay, the signal from the tape goes to the DSP with a different
>>>> configuration, then on to the firewire driver/contol circuits. It
>>>> doesn't
>>>> go anywhere near the CCD. So I fail to see that the CCD(s)
>>>> characteristics
>>>> can have any effect whatsoever during replay.
>>>> --
>>>> Tony Morgan
>>>>
http://www.camcord.info
>>>
>>>This I understand. I think the point I was making was that the bitstream
>>>carries less data in a single CCD camera than it does in a 3-CCD camera.
>>
>> I don't think so. On record, (with 3-CCD) the DSP resolves the 3 CCD
>> inputs to DV25 digital signal, feeds that to the drivers for the tape.
>>
>> On record with a single CCD, the DSP converts it to D25 digital signal
>> and feeds that (as before) to the drivers for the tape. Essentially the
>> same D25 signal, except that there might be a slightly lower quality
>> w.r.t. the 3-CCD operation.
>>
>> The sampling bit rate is exactly the same for both cases, and the D25
>> bit-rate is exactly the same also.
>>
>>>Forgive me if I misunderstand something here, but if the 3-CCD camera is
>>>capable of better quality images, then my logic tells me that each image
>>>frame must have more bits to describe it.
>>
>> I think you are looking at an analogue signal, where the bandwidth is
>> very significant. But here we are in the digital domain.
>>
>>> This would mean more bandwidth required and consequently better quality
>>> output circuitry than might exist in a single CCD camera. I have no
>>> idea if this is the case, however, and would welcome enlightenment.
>>
>> Perhaps the best way of understanding it, is to consider the process
>> where you use your camcorder to convert an analogue signal to a digital
>> D25 signal to record to tape. Most camcorders with DV-in allow you to do
>> this. I have "captured" analogue signals from both VHS tape (a quite low
>> quality - aka analogue bandwidth) and from my Sky box (appreciably better
>> quality than from VHS). The sampling bit-rate is the same for both. The
>> D25 digital signal recorded to tape is also exactly the same.
>> For both. The difference is not in bit rate (anywhere in the signal path)
>> but when ultimately decoded from digital to analogue, you are left with
>> essentially the same lower quality analogue (from VHS), and the higher
>> quality analogue (from the Sky box).
>>
>> For any particular sampling bit-rate, there is indeed a maximum analogue
>> bandwidth (aka quality) that can be encoded - but D25 encompasses both
>> VHS source and Skybox source within it's resolving range.
>>
>> A similar situation exists when comparing 3-CCD input/output with
>> single-CCD input/output. The sampling bit rate is the same for both, and
>> the D25 bitrate is the same for both.
>>
>> Three years ago, there was a more noticeable difference between 3-CCD
>> video and single CCD video, but the gap has closed considerably today -
>> especially with the upper-range single-CCD camcorders. Sony kicked it off
>> about three years ago with the development of their proprietary HAD
>> technology and their 16-bit DSPs. Prior to that all camcorder DSPs were
>> 12-bit wide and today the low-end camcorders still use 12-bit technology.
>> Because of the DSP bus width, they are able to do more real-time
>> processing to improve the quality of the image presented to the tape
>> recording drivers - but the recording always uses the same bit-rate of
>> D25. Taking analogies further, it's like comparing 16-bit PCs with 32-bit
>> PCs and (now) 64-bit PCs. All it means is that more processing can be
>> done in a given "window" of time.
>>
>> Another example might also illustrate it all. Consider the digital
>> storage of inputs from high quality microphones (with a dynamic range of
>> 20Hz to 20KHz) compared with low quality microphones (with a dynamic
>> range of 50Hz to 16KHz). The sampling rate of both is the same, as is the
>> bit-rate of the resulting signal. But when the digital signal is
>> extracted, there is the same difference in bandwidth (aka quality) as the
>> original analogue signals.
>>
>> Before someone jumps in as so often happens, I have here been discussing
>> in the context of miniDV camcorders, not the professional recording
>> technologies.
>
> Reading these posts gave me another idea about how to express the cheap
> vs. expensive problem here. This will clarify (I hope) or muddy the
> discussion ;-)
>
> Both cameras use the same sampling rate, the same number of samples per
> second, and the same digital tape format to create their digital data from
> the analog input. The cheaper camera might produce a poorer analog signal
> before it gets to digital, and might convert it less accurately, but both
> cameras have an equally precise encoding of their respective signals.
>
> Thus the expensive circuitry produces a very accurate rendition of a good
> signal and the cheap circuitry produces an equally accurate rendition of a
> poor signal.
>
> I'm fudging a bit here, in that poor A-to-D conversion means the cheaper
> camera produces a very accurate rendition of a converted signal, which is
> not necessarily the exact input to the A-D conversion.
>
> On analog playback the cheaper camera may also do more poorly, due to poor
> D-A conversion and/or poor analog circuitry after the conversion, but on
> digital transfer, you get the bits that are on either tape (the digital
> part was already said, in slightly different words, in this thread).
>
> Gino
>
> --
> Gene E. Bloch (Gino)
> letters617blochg3251
> (replace the numbers by "at" and "dotcom")
>
>
Thank you, gentlemen. It's all a lot clearer now.
Chas
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