Assuming you can sustain the maximum datarate for the nRF24L01 (2 megabits persecond), then that means you can move — in a perfect world — 200 kilobytes per second (assuming no overhead).
So given this, and your desired minimum of 24fps, you can calculate just how many bytes you need each image to be:
200K / 24 = 8.53K per frame
Now you haven't said what resolution you want, but the maximum resolution of the ov7670 is 640x480, and it uses 16bits per pixel (it is a little more complicated than that, I invite the curious to read the Data Sheet).
As our calculators all know
640 * 480 * 2 = 614,400 Bytes — 72 times that 8.53K per frame. In fact it would take in the ballpark of 3 seconds per frame (6 seconds if running at 1mbps).
So to answer your first two questions: The nRF24L01 isn't up to the task of transmitting live 640x480 video.
So this leaves us with your third question: How do reduce the size of the cameras data?
There are (not mutually exclusive) three ways of doing this:
- Compress the images
- Compress the data
- Send smaller images
Let us break each of these down:
Compress the images
You could, for example, send the images as a M-JPEG stream. This would certainly make decoding the images on the phone side much easier, and would reduce the size of the images sent quite a bit.
But there is one problem: You need to be able to hold the whole image in memory in order to do JPEG (and thus M-JPEG) compression. Your ST32F103RET6 has 64K of RAM (IIRC), so there is no way it is going to fit. And I am not sure of a lossy compression scheme you could use that doesn't need the whole image at once.
Compress the data
Now there are a number of options you could do here: Huffman, Run Length Encoding, LWZ, etc. Unfortunately none of these are going to produce a predictable amount of compression. It is going to depend on the images you send.
But I think it is safe to say you aren't going to get the 8.53K you would need for 24fps.
Send smaller images
The OV7670 is rather flexible when it comes to resolutions. So let us take a look at some other resolutions you could use:
- QVGA (320x240):
320 * 240 * 2 = 150K per frame. At this rate you could send just over 1fps
- QQVGA (160x120):
160 * 120 * 2 = 37K per frame. This is the first image size you could store entirely in RAM
- QQQVGA (80x60):
80 * 60 * 2 = 9.38K per frame. With compression you should be able to do 24fps video
- QQQQVGA (40x30): `40 * 30 * 2 = 2.35K per frame. At this (postage stamp) of a size you could steam at 30fps 1mbps! I believe this is the lowest resolution supported by the camera.
QQVGA may be possible if you do very lossy JPEG compression, and then do some compression on the data stream as well. You are going to have to experiment to be sure.
You are going to be hard pressed to find a faster wireless technology than the nRF24L01 (and its ilk), without going to wifi (for example the Adafruit CC3000 Module). With that, a microcontroller with a LOT of ram, and compression, you should be able to stream 24-30fps.
Alternately there are camera driver chips that do the JPEG compression for you — the vc0706 for example. With that attached to a camera, and using the vc0706's SPI link you should then be able to use even a trivial microcontroller to transmit the data.
I have yet to meet a camera module with vc0706 that exposes the SPI pins, they are all serial. One may exist, but I haven't found it yet. So if you go down this route, you may have to do it yourself...