Digitally Controlled AGC

AGC (automatic gain control) is nothing new and is used in a variety of equipment from CCTV cameras to fibre optic transmission equipment. When it came time to implement AGC in our Fibre Lite range of products we decided to do something really special with the AGC.

We analysed the sort comings of traditional Gain Control on analogue fibre optic equipment and found that the biggest problem was video drift. Simply put this is when the image starts losing its contrast, colour and eventually video sync (the image rolls on the screen). Video drift is caused by a number of factors like; cable aging, electronic component aging, large temperature fluctuations, additional losses on the cable due to the cable being damaged and requiring some splicing work, etc.

The Gain Control is there to compensate for all these variables but it’s normally a manual adjustment. This means that the system requires regular servicing by a qualified technician to maintain a crisp high resolution image.

We thought that in today’s world of high labour costs and the low cost digital microprocessors; why not have a microprocessor continually sampling, analysing and adjusting the gain control? This is how we come up with the idea of Digitally Controlled AGC. In our Fibre Lite range each fibre optic video receiver has its own dedicated microprocessor continually sampling, analysing and adjusting the gain control. Below is a simple diagram of how it’s done.

This is the magic behind Fibre Lite and what makes the product range adjustment free without digitally encoding the video as is the case with our Fibre Pro range.

http://www.bfrdigital.co.za/fibrelite.html

Can’t get Video Images from your IP Camera?

I've been asked on several occasions to assist CCTV and Networking companies resolve issues with IP camera installations. The common fault I have encountered is that the system can view the IP camera settings on a web browser but it’s not able to view the camera images.

The common problem I encountered on all these installations is that the maximum CAT5 cable distance between the IP camera and the Ethernet Switch has been exceeded. Here is a table that I recommend:

If your installation needs to exceed the 100m limit then consider one of the following options:

1. Repeat the signal by installing another Ethernet Switch before the recommended maximum cable distance is achieved.
   
2. Consider installing fibre optic cable and fibre optic media converters.

BFR Digital Ethernet Switches: http://www.bfrdigital.co.za/switches.html

BFR Digital Media Converters: http://www.bfrdigital.co.za/dtxseries.html

Ethernet Surge Protector

BFR Digital releases its new Ethernet surge protector.

Specifications:
Standard: 10/100Base-T
Nominal discharge: 5KA
Voltage limit: ≤ 40V
Response time: ≤ 1ns

Connectors: RJ45
Working temperature: -20 to 60°C
Not suitable for PoE applications

http://www.bfrdigital.co.za/

Twisted pair video transmission (UTP)

This contribution is courtesy of my colleague Renato Bordin.

Video signals from cameras are transmitted to the receiving end via coax cables, normally RG59, RG11 and so on. The camera’s composite video output buffer has a series output resistance of 75Ω and requires an end of line load impedance of 75Ω, hence all video equipment such as matrices, DVR’s, monitors etc have a cable termination of 75Ω to match the cable and camera specifications. This transmission method is referred to as an unbalanced signal path and requires a return path which the coax cable provides in the form of the screen or braiding. The concept is the same as a light bulb connected to a battery with two wires, remove one wire and the bulb stops burning since there’s no electrical connection to complete the current flow. This method of video transmission has one major disadvantage; the video is referenced to ground making it susceptible to common mode interference and provides a path for potential differences between two ground points. This often results in poor quality pictures received on the monitor and when a lightning ground surge moves between the camera and monitor the potential difference is now at a dangerous level resulting in camera failure. Other disadvantages are the attenuation properties of coax cables, RG59 for example cannot reproduce a high resolution image at 300m, the high frequency attenuation, and hence the upper end of camera resolution is reduced. The net result is a high resolution camera installed for a particularly demanding application but the received picture is in fact more a medium resolution image. Bear in mind that most applications involve a digital recording device that further reduces picture content or resolution. Digital recording devices require the highest possible camera resolution or rather video bandwidth to reproduce an acceptable image after digitization and then compression.

The alternative to using coax cable as a video transmission medium is the twisted pair cable but we have some challenges to overcome. Let’s take a look at the most popular of twisted pair cables – CAT5e, widely available and well known in the IT industry. For starters the cable impedance is 100Ω and not 75Ω as required by a camera and has no screen, so induced common mode interference becomes a problem. More importantly the cable is designed for a balanced or differential transmission method such as Ethernet and RS485/422. In the active domain differential input and output operational amplifiers (Opamp’s) are used to convert an unbalanced signal to a balanced or differential signal appropriate for twisted pair transmission and visa versa at the receiving end. In the passive world a Balun transformer is used to match impedances. In simple terms this device receives video from a camera and converts it to balanced video matched for a cable impedance of 100Ω and the receiving end converts this balanced video signal back to its original format suitable for a coax connection to the DVR, matrix etc. There are only advantages in using CAT5e cable for video transmission. 1. Since we now have balanced video, common mode interference is rejected far more than coax, resulting in less noise present on the picture. 2. CAT5e cable frequency response specifications are ideal for video so longer cable runs are possible. RG59 cable runs must not be longer than 220m – 250m using high quality cable but CAT5e cable runs can be at the 300m mark and up. 3. Since CAT5e has 4 twisted pairs, one cable can carry 4 video signals. 4. Some twisted pair video solutions have full ground loop isolation. 5. Installation ease and costs are less than a coax based installation. I’m sure several installers reading this can see several other installation related advantages.

The VIDEO CAT range of products available from BFR Digital currently includes the VC-01, a direct camera mount twisted pair converter and the VC-016R. This is a 16 channel rack mount version with full ground loop isolation. Both products feature BNC’s for camera or head end video connections and terminals for twisted pair connections.

Renato Bordin

http://www.bfrdigital.co.za/videocat.html

February 2009 fly-fishing trip

These pics are courtesy of my wife. They were taken earlier this year at Millstream Farm in Dullstroom. This is one of my favourite fly-fishing spots.

I had lots of luck with a dry line and an olive red-eyed damsel. I caught 19 rainbow trout that weekend but was only able to release 18.

Why AM modulated analogue multimode fibre optic video transmitters are smaller than digitally encoded multimode fibre optic video transmitters?

I think the best way to explain this is with a few block diagrams and some photos. The block diagram below shows the basic design of a simple AM modulated fibre optic transmitter.



Our first fibre optic transmitter released in 2002 ran on this exact design and below I include a photo of this product’s PCB (Printed Circuit Board).

As you can see the design is so simple that the product is a mere 35mm in length.

A digitally encoded fibre optic transmitter is more complicated because all the video and data signals have to be converted into digital signals before they are processed and transmitted. The block diagram below shows the basic system design for this type of product.

As you can appreciate this is a far more complex design. The picture below is of our VTXD-030-SMM. When we designed the VTXD-030-SMM we decided to make the video transmitter and the video/data transmitter a one PCB design and only populate the component required for the particular model. It is for this reason that our video and our video/data units are the same size. This was done to simplify the product range and to decrease the cost of the product.

This product is 133mm in length, which is 98mm longer than the simple analogue design.

http://www.BFRDigital.co.za/fibre.html

BFR Digital Advert

I am absolutely overwhelmed by this advertisement that our marketing team and our advertising agency put together. Please click on the advert above and have a look at it in its original size.

I think it’s fantastic. It delivers a simple and clear message.

Well done to both our marketing team and our advertising agency! :-)

Let me know what you think.

Field-Lite Termination Enclosure

This is one very cool piece kit!!! 4 multimode video transmitters, 1 multimode data receiver with power supply and splice cassette in one field enclosure.
When I first designed the Fibre Pro Field Termination Enclosure (FIELD-ENC) I never thought it would be as popular as it is. This popularity has led to the DTX-Adaptor, which is an adaptor plate to accommodate the DTX Series equipment in the FIELD-ENC.

Now with the Fibre-Lite Field Termination Enclosure (FIELD-LITE), I took a completely different approach. As you can see, I designed this product to be modular. The base of this product is a motherboard which includes the power supply for the transmitter/receiver fibre optic modules; 4 output data channels to control PTZ cameras and a 12-way splice cassette. On to this motherboard, up to 5 multimode fibre optic modules can be plug-in. The modules could be 5 video transmitters OR it could be 4 video transmitters and 1 data receiver. The data from the data receiver is distributed and offers one output per video channel. All this is housed in a IP55, self-extinguishing plastic enclosure according to IEC60695-2-1 standards.

http://www.BFRDigital.co.za/

Point-to-Point Asynchronous Transmission

The objective of this posting is to try and explain why our DTX Series of products do NOT require any IP addresses in the simplest way I can. The DTX Series range of BFR Digital products offers Point-to-Point Asynchronous Transparent Transfer of data.

Let’s break this down into 2 separate parts:

1. Point-to-Point

Point-to-Point is the simplest network topology and is merely a permanent network connection between two network end points. The simplest comparison is that of a tin-can telephone.


2. Asynchronous transmission

Asynchronous transmission is a data link protocol primarily used for point-to-point network topologies. Data is transmitted between the two network end points and an indefinite amount of time may lapse between the transmission of data packets. The data packets are prepared for transmission by encapsulating the data. This encapsulated packet of data is called a frame. Then a start bit is placed in front of the frame, this start bit alerts the receiver to the incoming data. At the end of the frame a stop bit is included and this tells the receiver that it is the end of the incoming data. The one step missing here is the portion with the error handling but for the purpose of this explanation we will ignore this step.

Let’s say that you want to transmit a data packet from IP address 192.168.1.10 to IP address 192.168.1.15 over a DTX fibre optic link. The DTX link will transmit all data presented to it regardless of the final IP destination. The IP address along with its data will be encapsulated into a frame and transmitted from one end of the link to the other. On the receiving end the encapsulation is removed and the destination IP address and its data are presented to the receiving end of the network in its original state. This is why the DTX Series of products do not require their own individual IP addresses and behave as though the link were just a simple CAT5 cable linking two network end points.


http://www.BFRDigital.co.za/

Laser Emitters

Earlier this month I was asked to assist a customer with a 32 link, DTX Series system. The customer failed to understand what transparent point-to-point transmission actually means and I will be addressing this in my next posting.

What I do want to talk about is safety precautions that need to be taken when working with fibre optic transmission equipment that use Laser Emitters. Both our Fibre Pro and DTX Series use laser emitters. These Lasers provide stimulated emission, i.e. they are on all the time.

The Fibre Pro and DTX Series products emit intense infrared light at 1310nm and 1550nm which is invisible to the human eye. This radiation can permanently damage the retina of the human eye. The diagram below shows the visible and invisible light spectrum.

All Fibre Pro and DTX Series products are packaged in antistatic bags that are sealed with a cautionary notice. Please read the notice before tearing the seal. The notice is there for your safety.

Please take the following precautions:

1. Never look into the source or the fibre cable.
2. Never power-up the source useless the fibre optic cable is connected to it.
3. Always keep the protector cap on the source if no fibre optic cable is connected.
4. Be cautious!!!

Fibre Pro


DTX Series

www.BFRDigital.co.za

Calculating - Fibre Optic Power Budget

Calculating the Optical Power Budget for a fibre optic cable link is really simple. Firstly, we need to start with some known facts. The losses quoted below are the norm but you can obtain the cable and connector losses from your local cable manufacturer for a more accurate calculation.

Let’s look at the following example:

1. Calculate the Cable Loss
Cable loss = Cable Distance x Cable Loss per km
Cable loss = 2km x -2.5dB/km
Cable loss = -5dB

2. Calculate the Connector Loss
Connector Loss = Connector Quantity x Connector Loss
Connector Loss = 2 x -0.5dB
Connector Loss = -1dB

3. Calculate the Splice Loss
Splice Loss = Splice Quantity x Loss per Splice
Splice Loss = 2 x -0.2dB
Splice Loss = -0.4dB

4. Calculate the Total Link Losses
Total Link Losses = Cable Loss + Connector Loss + Splice Loss
Total Link Losses = -5dB + -1dB + -0.4dB
Total Link Losses = -6.4dB

We have now successfully completed the first part of the calculation and we have established that the total loss on this fibre optic link is -6.4dB. The second part of the calculation is even simpler because most manufacturers of fibre optic transmission equipment quote the Optical Power Budget in their spec sheets. For example the Optical Power Budget for the Fibre Lite Range of products is -12dB. Alternatively, we can calculate the Optical Power Budget:

Transmitter Output Power (LVTXD-010-SMM Fibre Lite) = -14dBm (quoted by the manufacturer)
Receiver Sensitivity (LVRXD-010-SMM Fibre Lite) = -26dBm (quoted by the manufacturer)

Optical Power Budget = Output Power - Receiver Sensitivity
Optical Power Budget = -14dBm - (-26dBm)
Optical Power Budget = 12dB

Power Margin = Optical Power Budget – Total Link Losses
Power Margin = 12dB + (-6.4dB)
Power Margin = 5.6dB

It is good practice to allow for a Power Margin of 3 dB. This allows for cable degradation and should you ever have the misfortune of having the cable damaged or cut you still have enough Budget to re-splice the cable.


www.BFRDigital.co.za

Fibre Lite

We at BFR Digital have just completed our Fibre Lite product range. This range of products caters for the transmission of CCTV video signals and unidirectional data signals over multimode fibre optic cable. Fibre Lite offers an optical budget of 12dB; which means a maximum cable distance of 2.5km and a system safety margin (excess power margin) of 3dB.

This product range is the first in its class to feature Digitally Controlled AGC; this feature automatically adjusts the AGC to compensate for optical cable degradation and other insertion losses. Each video receiving channel has it own dedicate processor that continuously samples and adjusts the AGC.

In true BFR Digital fashion the Fibre Lite range is designed to be robust and easy to use; for this reason the Fibre Lite range is offered with a 2 year warranty. We achieve this robustness and reliability by over engineering our products.

www.BFRDigital.co.za