Frequently Asked Questions¶
What are the differences between the CMUcam1 and the CMUcam2?¶
The CMUcam2 includes all of the functionality of the CMUcam1 in an enhanced form and a lot more. Below we briefly describe the new functionality and the differences. You may also want to look at the answer to the question of how to decide whether to purchase a CMUcam1 or CMUcam2.
There are two main hardware differences which are important, the CMUcam2 uses a different processor than the CMUcam1 and the CMUcam2 incorporates a frame buffer chip while the CMUcam1 does not.
The CMUcam2 uses the SX52 processor and the CMUcam1 uses the SX28 processor, both are from the SX processor series from Ubicom. In both cases the processor runs at 75 MHz so there is no difference in processor speed or computational power. The advantages of the SX52 processor are that it has more RAM (262 vs. 136 bytes), more ROM (4096 vs. 2048 words), and more I/O pins. More RAM and ROM meant that we could write more complex code which allowed us to incorporate more functionality in the CMUcam2. The larger number of I/O pins meant that we had more pins left over for other functions - like more servos, more configuration jumpers, etc.
The big difference between the two systems is that the CMUcam2 includes the AL422B frame buffer chip from AverLogic while the CMUcam1 does not. This allows the CMUcam2 hardware to quickly capture a single complete frame and store it the frame buffer memory. This has a number of advantages:
- The CMUcam1 the processor had to process the pixel data stream on the fly as it is output by the camera module. This design helped reduce the complexity and cost of the system hardware. However, it limited the complexity of the processing because it had to be performed in real time between pixels or between image rows. Because the processor on the CMUcam2 accesses pixel data from the frame buffer at its own pace, it can perform more complex processing per pixel.
- The flip side of not having to stay synchronized with the camera module pixel stream is that the processor does not have wait for the camera module when it needs pixel data. Eliminating this waiting time means that the CMUcam2 processor can actually process a frame faster than the CMUcam1 can even though the processor is not any more powerful.
- Because the image stored in the frame buffer does not change, the CMUcam2 can perform multiple operations on a single frame. So for example, motion could be detected, region statistics could be computed, and multiple colors could be tracked all on a single frame.
- Because the CMUcam1 had to synchronize the transmitted frame dump data with the camera module data stream, it could only send one column of data per camera frame. This meant that different parts of the image were captured at different times, so quickly moving objects would be smeared across CMUcam1 frame dumps. Since the CMUcam2 transmits the image from the fixed data in the frame buffer this is no longer an issue.
- Having a frame buffer also means that the timing of the data transmitted by the CMUcam2 can be completely decoupled from the timing of the data read from the camera module. This means that CMUcam2 data from frame dumps and bitmap line modes do not change when the communications baud rate changes, the CMUcam2 will communicate the same frame or bitmap data to the host processor no matter the baud rate. However the number of frames processed per second may still change with slower baud rates because it still needs to eventually complete sending all the data from one frame before the following ones can be processed.
- So that the processor in the CMUcam1 could remain synchronized with the data stream from the camera module, the camera module frame rate had to be slowed down to 17 fps. The frame buffer that the CMUcam2 uses allows us to operate the camera module at the full frame speed. One advantage of this is that the automatic exposure and white balance adjustment of the camera can operate more quickly because it can only make a single adjustment per frame and now there are more frames per second. The second advantage is that the black and white analog video output (which only works when the module is operating at the full frame rate) can be used while the CMUcam2 is processing image data which was not the case with the CMUcam1.
The CMUcam2 implements all of the functionality of the CMUcam1 and also adds a lot of new functionality. The following is a summary.
- The CMUcam2 implements color tracking just as the CMUcam1 did. As in the CMUcam1, an optional bitmap line mode is implemented which transmits a bitmap of the tracked pixels. There are also some enhancements. There is a new optional line mode which provides statistics of tracked pixels for each row, including the mean, minimum and maximum positions of the tracked pixels. This is very useful for line following. There is also a new optional mode where the interpretation of tracking bounds can be inverted. In this mode pixels outside the tracking bounds are considered "good". This is useful for tracking an object against a homogeneous background - think blue screen special effects. It is also useful for detecting edges when pixel differencing (described below) is enabled. The noise filtering option has also been enhanced to allow adjustment of the amount of filtering for noisy situations.
- The CMUcam2 implements the computation of the mean and variation statistics of image regions just as the CMUcam1 did. As in the CMUcam1, an optional line mode is implemented which transmits the mean of each line of the image. There is a also a new enhanced line mode which optionally includes variation information on a line by line basis.
- Completely new in the CMUcam2 are motion detection (frame differencing) commands. These commands can be used to instruct the CMUam2 to capture a low resolution version of the current image and continuously compare this to new incoming images. Packets from the CMUcam2 report if any part of the image changes more than a specified amount, which would potentially indicate motion. These packets can describe the centroid and extent of the changed image blocks or provide a bitmap of which image blocks have changed. This mode can also be combined with pixel differencing (described below) to make motion detection more robust to changes in illumination.
- Completely new in the CMUcam2 are histogram computation commands. These commands can be used to capture a one dimensional histogram of a single color channel at a resolution of up to 28 bins. Histograms provide useful information which summarizes the appearance of an image. With additional programming on the host processor this information can be used to help in detecting obstacles, specific objects, or locations.
- The CMUcam2 implements the ability to restrict processing to a small region (subwindow) of the full image just as the CMUcam1 did. In the CMUcam2 this function is even more useful because it can be used to closely examine sub-windows of a single image stored in the frame buffer. Also, reducing the vertical window size can be used to greatly increase the frames processed per second by greatly reducing the number of pixels which need to be processed by the CMUcam2.
- The CMUcam2 implements much more flexible packet customization than the CMUcam1. There are commands that allow the user to customize each packet generated by each command to only return the values needed for a particular application. This can greatly decrease the amount of data transmitted to and processed by the host processor in many situations.
- Completely new in the CMUcam2 is an optional pixel differencing mode. In pixel differencing mode pixels are pre-processed before they are passed on to the rest of the CMUcam2 code. This pre-processing step sends new pixel values to the rest of the code which are the difference between the current pixel value and the previous one. Basically what this does is filter out everything except for vertical edges in the image. This is a very powerful operation and with additional programming on the host processor, can be used to aid in obstacle detection and line following.
- Completely new in the CMUcam2 is an optional down sampling mode. In down sampling mode the CMUcam2 software reduces the resolution of the camera image before processing it. The advantage of this is that there are many fewer pixels to process and transmit. Because it is done in software, down sampling results in only a very small increase in processing speed. The big benefit is in reducing the amount of transmitted data. In the case of a dumped image frame down sampling can easily reduce the data size and hence reduce the transmission time of by a factor of 2 or 4 or more. Similarly when bitmap line modes are used the size of the bitmap image can be greatly reduced, decreasing the amount of data that a host processor has to receive and process.
- The original CMUcam1 supported only a single servo and that servo would only operate properly at specific baud rates and when the CMUcam1 was in streaming mode. The CMUcam2 supports up to five servos. The servo controller code in the CMUcam2 is implemented as a background process so the servo outputs always remain stable and it can be used as any other servo controller would be used. Additionally, the CMUcam2 can be configured to automatically control pan and tilt servos. In this mode the CMUcam2 will update the servo positions each time it is commanded to compute color tracking data.
- Completely new in the CMUcam2 are power saving modes. In some applications the CMUcam2 is not required to continuously process image data. In these cases the CMUcam2 can be commanded to go into a variety of power down (or sleep) modes. No tracking commands or servo control will happen when the CMUcam2 is in one of these modes. Sending a simple serial command wakes the camera back up in a few milliseconds.
What is the difference between using the OV6620 and the OV7620 module with the CMUcam2?¶
In terms of capabilities, the OV7620 sensor is a higher resolution sensor (664x492 raw sensor locations) than the OV6620 sensor (356x292 raw sensor locations). But this fact has very little to do with the issues which arise when considering which of these sensors best matches your application of the CMUcam2. Following are a list of considerations.
While it is true that the OV7620 sensor is a higher resolution sensor than the OV6620, because of the fixed memory size of the frame buffer of the CMUcam2, the CMUcam2 only supports a single resolution of 160x239 for the OV7620 sensor. The CMUcam2 supports a low resolution mode of 88x143 and a high resolution mode of 176x255 for the OV6620 sensor. So using the OV6620 in high resolution mode one can actually achieve higher resolution operation that the single resolution mode available for the OV7620 sensor.
Another resolution related issue is maximum frame rate when continuously processing a stream of images. The lower the resolution, the fewer the pixels which need to be processed and the higher the achievable frame rate. Although an increase in frame rate can be achieved by changing the down sampling rate and the virtual window size, the actual number of pixels output by the sensor has a much larger effect. Because of this, the CMUcam2 can achieve a much higher frame rate when the OV6620 is operated in its low resolution mode than the single resolution mode available for the OV7620.
Analog Video Output¶
Another difference between the two sensors is the format of the analog video output of the two sensors. The OV6620 sensor outputs in analog PAL format and the OV7620 sensor outputs in analog NTSC format. In both cases the output is in black and white.
It is important to note that in most uses of the CMUcam2 the analog output will not be used. In a typical application the results of processing the image or the raw pixels are transmitted digitally via the serial port to the host computer or microcontroller so this is not an issue. However, if live black and white video is important, then you may want to take the format of the analog video output into account.
Our recommendation is that the OV6620 module is the best choice for almost all applications, especially considering its lower cost and the faster processing time achievable with this module. In the rare applications where NTSC monochrome analog video output is required you may want to consider the OV7620.
Are there any known issues with the current CMUcam2 firmware releases?¶
Yes. In the 1.00 release the "RM" (Raw Mode) command did not work correctly. The 1.01 release fixes that issue.
The firmware in the CMUcam2 is upgradeable, but an SX-Key (or compatible) programmer is needed to do so. The "hex" files for the ROM images are available on the downloads page.