Point Grey Grasshopper - Exceeding Bandwidth Limitations Using Format_7 with Multiple Cameras; Calculating Maximum Possible Frame Rate; Dual Packet Mode

Point Grey Research Grasshopper Technical Reference
Camera Operations and Features
Revised 26-Nov-10
Copyright (c) 2010 Point Grey Research Inc.
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• The 1394b standard limits the maximum number of devices on a single bus to 63.
• An inadequate power supply. Consult the voltage and power requirements in the General
Specifications section to determine the amount of power required to operate the cameras
effectively.
4.3.2. Exceeding Bandwidth Limitations Using Format_7 with Multiple
Cameras
There is a mechanism for effectively bypassing IEEE-1394 bus bandwidth negotiation when using
cameras in Format 7 partial image mode. This functionality is useful in any situation where the
user is trying to host multiple cameras on the same bus in a configuration that would normally
exceed the bandwidth allocation, but where the cameras are configured to transmit data in a
manner that does not exceed the total bandwidth. For additional information, see
Knowledge
Base Article 256.
4.3.3. Calculating Maximum Possible Frame Rate
The maximum frame rate allowable for each of the cameras on the bus depends on the resolution
of the cameras and the frame rate, and can be roughly approximated using the following general
formula (assuming all cameras are at the same resolution):
Frames_per_second = (Bandwidth / (Pixels_per_frame * Bytes_per_pixel)) / Num_cameras
Example:
To calculate the approximate frames per second available to three 1024x768 Grasshoppers that
are in 16-bit mode, you would calculate:
Frames_per_second = (80MB/s / (1024*768*2bytes/pixel)) / 3
= (80MB/s / 1.5MB/frame) / 3
= 53.33 FPS / 3
= 17.8 FPS
The calculation above is only a rough estimate. The DCAM standard defines a specific number of
bytes per packet (BPP) for every non-Format_7 video format/mode/frame rate combination. This
number is generally higher than the minimum bandwidth that might be expected. In order to
accurately determine whether or not there is enough bandwidth available for a given scenario,
these numbers must be used. The BPP can be derived using the Isochronous Bandwidth
Requirements section of the PGR IEEE-1394 Digital Camera Register Reference.
For example, a single Grasshopper in 640x480 RGB mode running at 15 FPS is sending 640
pixels per packet. Each pixel consists of 24 bits, or 3 bytes, of data. Therefore, the camera is
sending 640*3 = 1920Bpp of data. The maximum bandwidth of the 1394b bus as discussed
above is 8192Bpp, so it would be possible for 8192/1920 = 4 (rounded down) Grasshopper’s to
run in 640x480 RGB mode at 15 FPS on the same 1394b bus.
4.3.4. Dual Packet Mode
The Grasshopper is capable of sending a maximum of 9568Bpp (8-bit 5MP images at 15fps).
However, the 1394 specification only allows 8192Bpp, so the Grasshopper splits each packet that