Curiously, 1080 60i and 720 60p come out nearly the same size.

If they are the same bitrate and length, they would be near the same size.

I'm still trying to absorb this. Keep thinking about it.

I know you are correct but isn't 1920x1280 pixels a bigger number than 1280x720 pixels?

Can you explain why the two files are the same size? With a still image like a jpg, the 1920x1280 would be much large.

Once again, I know you are correct, I just would like to understand this.

Thanks,

As DonaldT explained it's the bitrate that counts. This is independent of the image size. So with a larger image area the bits per second are spread out over a larger area.

Jpg's are different as they aren't video with a bitrate based on time.

Video has three dimensions, frame width, frame height and time. Still photos only have two dimensions, width and height. The encoders in Vegas typically allow one to encode to a target bit rate. A target bit rate could be expressed as 10,000,000 bits per unit time or 10 Mbps. The 10 Mbps budget is distributed across the frame size dimension and the time dimension. A measure of that distribution is exposed in Mediainfo as Bits/(Pixel*Frame).

One thing to keep in mind when comparing 1080-60i and 720-60p is though 1080-60i has a larger frame size the frame rate is 29.976 frames per second displayed as 59.94 interlaced fields per second while 720-60p has 59.94 frames per second.

The larger pixel dimensions of 1920x1080 yields 2,073,600 pixels per frame and when multiplied by 29.976 fps there are 62,158,233 pixels per second that have to be described by our target bit rate. At 10 Mbps each pixel would get a fractional Bits/(Pixel*Frame).

The smaller pixel dimensions of 1280x720 yields 921,600 pixels per frame and when multiplied by 59.94 fps there are 55,240,704 pixels per second that have to be described by our target bit rate.

62,158,233 pixels per second is only about 12.5 % more than 55,240,704 pixels per second even if it did make any difference in the final file size.

I rendered the same one minute of video to 1080-60i and 720-60p at a constant target bit rate of 10 Mbps. The file sizes are within 2% of each other while the Bits/(Pixel*Frame) measurement varies by ~15%. I'll include the Mediainfo reports.

1080-60i.mpg
Format : MPEG-PS
File size : 79.6 MiB
Duration : 1mn 0s
Overall bit rate : 11.1 Mbps

Video
ID : 224 (0xE0)
Format : MPEG Video
Format version : Version 2
Format profile : Main@High
Format settings, BVOP : Yes
Format settings, Matrix : Default
Format settings, GOP : M=3, N=15
Format settings, picture structure : Frame
Duration : 59s 993ms
Bit rate : 10.5 Mbps
Maximum bit rate : 9 800 Kbps
Width : 1 920 pixels
Height : 1 080 pixels
Display aspect ratio : 16:9
Frame rate : 29.970 (30000/1001) fps
Standard : Component
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Interlaced
Scan order : Top Field First
Compression mode : Lossy
Bits/(Pixel*Frame) : 0.169
Time code of first frame : 00:00:00:00
Time code source : Group of pictures header
GOP, Open/Closed : Open
GOP, Open/Closed of first frame : Closed
Stream size : 75.2 MiB (94%)
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709

Audio
ID : 192 (0xC0)
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 2
Duration : 1mn 0s
Bit rate mode : Constant
Bit rate : 384 Kbps
Channel(s) : 2 channels
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 2.75 MiB (3%)

720-60p.mpg
Format : MPEG-PS
File size : 81.3 MiB
Duration : 1mn 0s
Overall bit rate : 11.4 Mbps

Video
ID : 224 (0xE0)
Format : MPEG Video
Commercial name : HDV 720p
Format version : Version 2
Format profile : Main@High
Format settings, BVOP : Yes
Format settings, Matrix : Default
Format settings, GOP : M=3, N=15
Duration : 59s 993ms
Bit rate : 10.8 Mbps
Maximum bit rate : 10 000 Kbps
Width : 1 280 pixels
Height : 720 pixels
Display aspect ratio : 16:9
Frame rate : 59.940 (60000/1001) fps
Standard : Component
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Scan type : Progressive
Compression mode : Lossy
Bits/(Pixel*Frame) : 0.195
Time code of first frame : 00:00:00:00
Time code source : Group of pictures header
GOP, Open/Closed : Open
GOP, Open/Closed of first frame : Closed
Stream size : 76.9 MiB (95%)
Color primaries : BT.709
Transfer characteristics : BT.709
Matrix coefficients : BT.709

Audio
ID : 192 (0xC0)
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 2
Duration : 1mn 0s
Bit rate mode : Constant
Bit rate : 384 Kbps
Channel(s) : 2 channels
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 2.75 MiB (3%)

With a JPEG you are talking compression. With video, you are talking bitrate. The amount of bits per second of video. If you have 10 seconds of video at Z amount of bits per second, regardless of the size of the frames, you still only have Z x 10.

If you wanted to make a comparison to compressing a jpeg file, then think of bitrate is similar to the total file size. If you had a 1920x1080 image and a 1280x720 image and you specified that both will be 1MB in file size then you'd have to compress the larger file more than the smaller file.

Bitrate is similar. You specify total bits per second. If the image frame is smaller then you don't have to compress it as much to meet your bitrate specification. But both files have the same bits/second so they're the same size in bits.

Rob

There really should be a sticky for this. This question seems to be asked in some for or other more often than any other topic. It also seems to cause more avoidable confusion amongst newcomers than anything else.

It has very much to do with interlace frames (fields) and progressive frames,
interlace video has 540 lines to a field,
progressive video has 1080 lines to a frame,

So when you render high resolution interlace 1920-1080i and low resolution 1280-720p progressive video the file size will be more or less be the same.

So when you render high resolution interlace 1920-1080i and low resolution 1280-720p progressive video the file size will be more or less be the same.

Only if the bitrate is the same.

So if I shot 1920 by 1080, 29.970 frame rate what should my render settings be for YouTube to minimise YouTube's recoding time. Thanks

I wouldn't worry about youtube's recoding time. The bit rate of your file is only one factor among others such as their system utilization when you happen to upload the file. I would focus on the quality of the video and the amount of time it takes you to upload it and let youtube handle their end of the transaction.

youtube's recommended file specs

Search youtube for "IMG" and you'll find plenty of folks that upload with little thought to youtube's encoding servers.