What is the advantage of capturing using A SDI card as opposed to software only ?
 

As a hobbiest I have been shooting HDV for a couple of years and using Vegas Video and Connect HD for the capturing program. I realize there is a fairly large expense for a SDI card ( approx 1000.00) but is that really necessary. In terms of video quality is there that much difference ?
I thank you for this forum and really enjoy learning more about videography

No not really.

There can be subtle differences in the signal processing chain if you come in via SDI versus firewire, since you end up using different codecs. It would be different in a pretty subtle way... sometimes better, sometimes worse. In the case of HDV, going in via connect HD would have the advantages of:
A- I don't believe you will encounter the black frames bug in Vegas.
B- It does chroma filtering for the 4:2:0 material. It is technically incorrect IMO, but it looks good.
C- The wavelet compression sort of removes a little noise in the image. (Though it also adds its own compression noise.)

If you go over SDI, the HDV image has to be resampled up to 1920x1080 (or 1280x720)... the resampling can have subtle artifacts and incur generation loss. So in this particular case you might want to avoid that.

It depends on your camera and workflow. There are not many HDV camcorders that have the SDI option. Only two that I can recall, the Canon G1 and JVC 200/250. If you don't have those cameras, SDI is not an option. If you do have one of the cameras I mentioned above, SDI has a huge advantage if you're taking the live signal out of the camera (uncompressed and 4:2:2 color space); however, the recorded HDV video doesn't give you much advantage whether digitizing from firewire or SDI. SDI does have the capability of capturing higher quality 10 bit video; whereas, firewire and HDMI (consumer equivalent of SDI) are only 8 bit, so the quality or recorded material might be a bit better.

Basically, projects that demand higher quality video require capture from an SDI card. Then, again, you wouldn't be using HDV for those projects.

It is only the JVC HD250 that has SDI. The other popular HDV cam not mentioned is the XLH1.

Canon XL H1 is only partial-spec SDI, as it carries video only, no audio, no TC.

Canon XH G1 is full-spec SDI, with not only video but also audio and TC in that pipe.

Of the JVC Pro HD series, indeed it is only the HD250 that has SDI and yes it is full spec.

My bad... thanks for clarifying.

Thank you all for the posts.