Standard DVD is very compressed. If it wasn't compressed, you'd only be able to fit about 6 minutes on a double-layer commercial DVD or about 3 minutes on a recordable DVD. The usual compression rate used for commercial DVDs would probably allow about 90 minutes on a standard 4.7GB disc, which would place it about halfway between the HQ and SP modes you've got listed.

the "sonic mydvd" program is set to a 120 min compression. is it possible to change this? i don't see an option in the menu, so me thinks no. which of the listed compression rates compares to vhs?

(btw russian, what you say??)

I think it goes like this:

2 megabits/sec = 4 hours or 1 minute = 15 meg

4 mbits/sec

Are you using the Sony DRU-500A Internal DVD±R/RW Drive? I am thinking of ordering one for my second PC, instead of an A04 or A05.

I think encoding rates in VHS terms go like this:

2 Mbits/sec = 300 minutes or 1 minute = 15 meg SLP

4 mbits/sec = 150 minutes or 1 minute = 30 meg LP

6 mbits /sec = 100 minutes or 1 minute = 45 meg SP

8 Mbits/sec = 75(?) minutes or 1 minute = 60 meg High quality (Direct connection from AVID output to capture board input for encoding best quality clips made from BetaSX)

peter

I'm using the Sony, but I think it's the 500UL. Same as the 500A, but external.

The subject of MPEG2 compression bitrates is very complicated, but I found a very helpful explanation in the Sonic MyDVD help file (this is, however, the ONLY thing I have found worthwhile about this software):

Choosing bit-rates for DVD

The video bit-rate is one of the most important encoder settings, affecting both the video quality and the amount of video you can fit on a disc. The bit-rate is the number of digital bits used to encode each second of video, and is usually measured in megabits per second (Mbps).

There are several factors that affect your choice of bit-rate. One factor is the type of disc you will use and, therefore, how many bits are available. Another factor is the maximum bit-rate allowed by the DVD-Video format. This is 9.8 Mbps, which must be divided between the video and audio streams. The final factor is how good you want the encoded video to look; if your video has fast action sequences, a low bit-rate may result in poor-quality video during those sequences.

The following can help you to select a bit-rate that gives the best results for a given video duration:

CBR (constant bit-rate) encoding and VBR (variable bit-rate) encoding are two options for encoding video.

CBR encoding allots the same number of bits to every second of video and requires you to specify a single bit-rate. The encoder uses this rate regardless of the video's complexity, so quality worsens as the video becomes more complex, and bits are wasted when there is not much happening.

VBR encoding requires you to specify a maximum bit-rate, a target (average) bit-rate, and a minimum bit-rate. The encoder uses higher bit-rates for complex sections and lower rates for "easy" sections, constantly adjusting to keep the overall bit-rate to the target value. VBR encoding can provide the same quality as a CBR encode but at a lower overall bit-rate (so you can fit more video on the disc), or better quality than a CBR encode at the same overall bit-rate.

Note: A good VBR encode requires two passes through the video - one to analyze the material for complexity, the other to do the encoding. This means that the process can take twice as long as a CBR encode.

VBR encoding works best for long video durations (100 minutes or more on a 4.7 GB disc). For shorter durations the difference in quality between CBR and VBR is usually not detectable. You can try encoding short "difficult" sections (sudden scene changes, or fast movement) using both methods to see if the difference is worth the extra effort.

Bit-rate calculation

Calculate the average bit-rate for both video and audio, by dividing the disc size (in megabits) by the total video duration (in seconds). See Recordable disc specifications for disc sizes:

Disc size: __________ Mbits Duration: _________ seconds = __________ Mbps

For example, to fit 100 minutes (6,000 seconds) of video onto a 4.7 GB DVD:

Disc size: 36096 Mbits Duration: 6000 seconds = 6.0 Mbps

Calculate the average video bit-rate by subtracting the audio bit-rate from the result of step 1:

Average bit rate: ________ Mbps - Audio bit-rate: ________ Mbps = __________ Mbps

The audio bit-rate depends on whether you will create a DVD or cDVD. For DVD discs, the (PCM) audio bit-rate is 0.8 Mbps for mono and 1.6 Mbps for stereo; for cDVD, the (MPEG) audio bit-rate is 0.096-0.448 Mbps. In our example, with a stereo soundtrack:

Average bit rate: 6.0 Mbps - PCM audio bit-rate: 1.6 Mbps = 4.4 Mbps

If you were using CBR encoding, you would enter 4.4 Mbps into your encoder's bit-rate field. If you were using VBR, you would enter 4.4 Mbps as the target bit-rate.

(For VBR encodes only) Calculate the maximum video bit-rate by subtracting the audio bit-rate from the maximum allowed by the DVD format:

9.8 Mbps - Audio bit-rate: ________ Mbps = __________ Mbps

Again, the audio bit-rate depends on whether you will create a DVD or cDVD. In our example:

9.8 Mbps - PCM audio bit-rate: 1.6 Mbps = 8.2 Mbps

So you could enter 8.2 Mbps into your encoder's maximum bit-rate field.

Note: Both CBR and VBR encodes can have transient "spikes" that go higher than the set bit-rate; if a spike pushes the total bit-rate above 9.8 Mbps, MyDVD will consider the file "illegal" and re-encode it. We therefore recommend that you always make the video bit-rate 8 Mbps or lower.

What about the minimum bit-rate?

The minimum bit-rate in a VBR encode should be about half the target bit-rate. In the example shown above, a minimum bit-rate of about 2 Mbps would be sufficient.

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Here's what I make of the above explanation:

Most encoding software gives you templates (calling them SP, LP, EP or maybe GOOD, BETTER, BEST -- something like that). These templates allow you to encode without doing any calculations. In many cases, you don't have access to the underlying parameters, and can't tweak the compression to suit your material. This is really a one-size-fits-all approach. I have had VERY BAD experiences (ugly compressed video) using such a cookie-cutter approach.

With Vegas (and Batch Converter), you can control all of these parameters and tweak your compression to your heart's delight. What is not noted in the article is that DVDs do not have to have the same compression scheme or bitrate all the way through. So complex scenes can be encoded at a high bitrate, and simple scenes at a much lower bitrate, and they will coexist just fine on the finished disc.

Another important thing to remember is that CBR encoding is MUCH FASTER than VBR encoding. If you have to have the project done fast and can afford the drop in quality, CBR may be a good option. VBR almost always yields better results in smaller files, but it takes time (up to 4 times as much time as CBR in my tests).

To implement the calculations in that article, I created the following templates for rendering/encoding in Vegas. When I encode, I usually pick a test section of about a minute, then try a few options before encoding the whole thing. (To do it right, you really would have to do this for each section of video individually, but who has time for that? -- unless the client is paying for it, of course...) The parameters you need to adjust are on the Video tab of the Customize dialog for the Main Concept MPEG2 encoder.

DVD NTSC 90 minutes - VBR, Max: 7,500,000 Avg: 5,000,000 Min: 2,500,000
DVD NTSC 120 minutes - VBR, Max: 6,800,000 Avg: 3,400,000 Min: 1,700,000
DVD NTSC 146 minutes - VBR, Max: 4,200,000 Avg: 2,500,000 Min: 1,250,000

I start with one of these templates based on the length of the program, then look for ways to squeeze a little or expand a little depending on the content.

When encoding with VBR, lowering the Video Quality slider speeds things up, with a corresponding loss of clarity. There are some times when this doesn't matter, but frequently you'll see a difference. Though it doesn't say so, I think this slider may determine how many passes the encoder does.

Hope this helps.

Rick
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What I dont understand with VBR is how does the dvd player constantly know how to adjust the rate it reads bits. I mean, wouldnt it need to read it at the exact rate it was put onto the disk? How is this done?

PBE, the DVD player is capable of reading the data from the disc much faster than is necessary for playback. It will buffer a certain amount of data ahead of what it's currently playing, then assemble the video stream out of this buffered data one frame at a time at the correct frame rate. Each frame may consist of a different number of bits. When more data is needed (or hopefully before it's needed), the player will read more from the disc.

The advantage of VBR is that more bits can be used for frames in scenes that have more action or detail without requiring the same amount of bits to be used where they're not needed and would be wasted.

one thing to remember with bitrates is that the DVD format has a maximum of about 10Mbps including sound (i think). This is important to remember if your doing multi angle or multi sound authoring.