stereo_synthesizer

Q1 is used as an emitter fol­lower buffer stage which ensures that the two filter networks fens from its output are driven from a low impedance source. If these were driven direct from the input, it is quite possible that they would be fed from a source impedance of a few kilo ohms or more, which would be quite sufficient to alter their effective characteristics.

The two filters are formed by R4 and C3 (low pass), and C6 plus R8 (high pass). A high roll off rate is by no means essential in this application and the 6dB per octave attenuation rate of simple RC filt­ers such as these is perfectly ade­quate. The -3dB point of each filter is at approximately 800Hz and the combined output of the filters, therefore, gives a virtually flat res­ponse with no significant peaks or troughs.

Q2 is connected as an emitter follower buffer stage and this ensures that there is minimal loading on the low pass filter. Q3 similarly ensures that there is minimal load­ing on the high pass filter, but this device is also used as a phase splitter. With SW2 switched to take the output from Q3’s emitter, Q3 effectively operates as an emit­ter follower and gives no phase inversion. With SW2 switched to take the output from Q3’s col­lector, Q3 then effectively acts as a common emitter stage with 100% negative feedback (and unity voltage gain) due to R11. 1t also provides a 180° phase shift so that the two output signals are in anti-phase. An in-phase relationship is needed to give a good central ste­reo image and the use of anti-phase signals tends to give an im­pression of increased channel separation.

In a stereo orchestral recording, it is normal for the violins to come from the left hand channel, with the cellos and basses from the right hand channel. Therefore, the high frequency signals are fed to the left channel and the low fre­quency signals are fed to the right channel so that the unit provides a similar effect (although it will ob­viously function properly with the outputs connected either way).

 Relying on the penetration of the GSM and 3G radio signals from the outside for providing in-building coverage is neither practical nor reliable in many buildings, and additional indoor antenna units (AUs) must be installed. Different building shapes and materials such as steel and metalized glass, cause in-building penetration of radio frequency (RF) signals to weaken resulting in reduced data rates and even complete loss of signal. This will become even more apparent over the next few years with the introduction of new 4G technologies which require considerably higher signal integrity and radio frequency than their voice counterpart. Also, with the trend to increasing data rates on wireless networks and a rapidly increasing number of users who want un-tethered access pose new challenges to system integrators and network designers. This results in significant challenges for the wired infrastructure that is required to connect the numerous antennas units.

 Wireless over fibre (WoF) systems which also known as Radio over Fibre (RoF) systems have attracted much interest for broadband wireless access, offering a simplified  overall system design due to the aggregation of RF signal generation and network management at a central location. Distributing broadband wireless signals over optical fibre has a number of advantages compared with traditional copper cabling. The wide bandwidth of optical fibre further allows different services such as Gigabit Ethernet, IEEE802.11a/g, GSM and 3G to share the same infrastructure, making the wireless-over-fibre approach a truly multi-operator and multiservice technology.

WLANs have had a profound impact on our perception of communication. First of all, the vast majority of users now believe in the new notion of “always on” communication. We are now living in the era of ubiquitous connectivity or “communication anytime, anywhere, and with anything”. Secondly, the concept of broadband communication has caught on very well. As fibre penetrates closer to the end-user environment (Fibre to the Home/Curb/X, FTTH/C/X), wired transmission speeds will continue to rise. Transmission speeds such at 100 Mbps (Fast Ethernet) are now beginning to reach homes. The demand to have this broadband capacity also wirelessly has put pressure on wireless communication systems to increase both their transmission capacity, as well as their coverage.

Figure 1: PRESENT COMMUNICATION SYSTEMS

In general there is a trade off between coverage and capacity. Figure 1 shows the relationship between some of the various standards (present) in terms of mobility (coverage), and capacity. For instance, the cell size of Wireless Personal Area Networks (WPANs) is typically a few metres (picocell), while their transmission rates may reach several tens of Mbps. On the other hand 2G (e.g. GSM), and 3G (e.g. Universal Mobile Telecommunication System (UMTS) and the International Mobile Telecommunications (IMT2000)) systems have cells that extend several kilometres, but have data rates limited to less than 2 Mbps. Therefore, as mobile communication systems seek to increase capacity, and wireless data systems seek to increase coverage, they will both move towards convergence. A case in point is the IEEE 802.16, otherwise known as WiMAX, which appears to lend weight to the notion of convergence, as shown in Figure 1 WiMAX   seeks  to  provide  high-bit  rate mobile services  using  frequencies  between      2-11 GHz. In addition, WiMAX also aims to provide Fixed Wireless Access (FWA) at bit-rates in the excess of 100 Mbps and at higher frequencies between 10 – 66 GHz.

Toshiba Satellite L300

Sometimes we get a notebook that packs so much punch into an affordable platform that makes us wake up and actually take notice. And this is the case with Toshiba Satellite L300D – 01N Laptop computer. Big name but small price!

This Satellite L300 laptop from Toshiba is well within an affordable price range, but it still packs all the useful components you need to get your life fun around.

What it has is Dual Core processing, Wireless connectivity and much more.

Processor

Toshiba Satellite L300 runs on AMD Athlon 64X2 Dual-Core QL-60 1.9 Ghz Processor. This CPU gives you processing speeds of 1.9 Ghz per core.

Memory

AMD Athlon 64X2 Dual-Core QL-60 1.9 Ghz Processor of Toshiba Satellite L300 is backed up by 2GB of DDR2 memory. This will knock out memory hungry applications easily. As well as making multi tasking operations.
Display

This Toshiba has a 15.4-inch WXGA diagonal LCD screen. It has maximum resolution of 1280X800 pixels. And since it is wide screen its perfect for viewing a number of documents at the same time, or navigating over a number of windows at the same time and of course watching DVDs.

Webcam

An onboard webcam is located right over the top of the screen. This gives a nice look of course.

HDD

Now let’s have a look at the hard drive space you have for the Toshiba satellite l300. Of course you need a lot of space to store all your data on the hard drive. This baby comes with a 160 GB HDD space for storing all your favourite applications, music, videos, data files etc.

Dual layer DVD drive

Now, in case you want to download all your favourite flicks, there is also a super multi double layer DVD drive. That means you watch DVDs, write all your data on to a dual layer DVD which has got around 8GB storage per disk!

Card reader

There is also a five in one media card reader, so you can upload and download all your media files in between your Toshiba satellite l300 and 5 different types of media cards.

Graphics and audio

Graphics and audio get a significant boost with this brand new computer. The Integrated Graphics ATI Radeon 3100 of Toshiba Satellite l300 produce brilliant visuals and on screen special effects, fantastic clarity and of course nice contrast.

Now, this laptop is also setup with nice audio technology, and it has support for MIDI and HD audio. All these will help you to listen to your favourites with crystal clear clarity.

Connectivity

The new Toshiba Satellite l300 has got 802.11 b/g wireless connectivity. In case if you have no wireless connections available you can make use of the Ethernet card for surfing the web. You also have 3 USB 2.0 ports. Express card slot is also available. Audio i/o is provided in the front for connecting your speaker/headphone. Toshiba Satellite could be connected to and external display device via the VGA port available.

Operating system

The whole system runs on Windows Vista Home Premium, one of the most popular operating system in the world today. That is the new operating system from Micro Soft, which is suitable for dual core high memory systems.

One of the major causes for sudden battery drain is nothing other than playing games. This fact might make you unhappy, but if you are going to a place where there is not facility to charge your mobile phone you can think of not playing games.

If you are sure that you are not going to make any calls and there is no chance for an incoming call, then switch off your mobile phone without a second thought. Thing is that, we usually keep our phones in our coat or bag in the switched on state, even when we are sure that we don’t need mobile phones for a specific time period.

If you are in an area of weak signal strength, more battery charge will be utilized, while your mobile phone searches for signal. So you can think of switching off your mobile phone if you are sure that you will be in an area of weak signal strength for a long period of time. At the same time less battery charge is used when your mobile phone searches in a strong signal condition.

You can get more out of your mobile phone battery if you turn off the backlight and vibrate function. Ringing takes less power as compared to vibrate function.

Limited net surfing can save battery charge. Sensible use of Bluetooth will definitely help you to save battery charge.

The preamplifier stage of the unit is based on IC1 which is a low noise op amp having a FET input stage. This is used in the conventional inverting audio amplifier mode and the negative feedback network, R1, 4, sets the voltage gain at about 40dB. (100 times C11 reduces the gain slightly at high frequencies in order to obtain an improved signal to noise ratio.

C4 couples the output from the preamplifier to volume control, RV1, and from here the signal is coupled to the power amplifier by C5. The output stage uses the TBA820M, a class B amplifier which will give an output power of a few hundred milliwatts rms. The closed loop voltage gain of the device is determined by the value of R6, about 25dB. (180 times) with the specified value. This gives the required very high overall gain in conjunction with the preamplifier’s gain. C7, R7 and C8 are needed in order to maintain stability.

The quiescent current consumption of the unit is only about 5mA, but this rises to as much as 50mA or so at high volume levels. The best position for the pick-up coil on the telephone base (not the handset) can be located with a little experimentation.

R1 and R2 are used to bias the unit to give the optimum quiescent output potential and they provide overall negative feedback, which improves the quality of reproduction. D1 and C4 are boot strapping components, enabling the gate drive voltage to Q3 to go above the positive supply potential, giving improved efficiency to the circuit. R3 is the main collector load for Q2 and PR1 is used to give a standing bias on the output transistors that gives a quiescent current consumption of about 25mA. The thermal compensation circuitry normally used is totally unnecessary in this circuit, since VMOS devices do not suffer from thermal runaway. In fact the quiescent bias current will drop slightly as the output devices heat up, but not sufficiently to give rise to significant crossover distortion.

C2 and C5 provide DC blocking at the input and output respectively, while C1 is a supply decoupling component. C3 gives a degree of high frequency attenuation and aids the stability of the circuit.

Although the current in the driver stage, only about 1mA, may seem to be totally inadequate, it is in fact more than sufficient since the VMOS devices have extremely high input impedances and consume no significant, input current. This is one of their main advantages over bipolar devices. One disadvantage in this particular application is lower efficiency due to the higher threshold voltages and on resistance of VMOS transistors in comparison to bipolar devices. However, the circuit will give an output of 10W rms using a supply voltrage of about 33V or so (with a current drain of up to about 600mA). An input of about 500mV rms is needed for maximum output.

Note: The output devices do not have internal zener protection diodes and the appropriate handling precautions should be taken.

The output signal level from a cassette tape head is typically about 500 micro Volt or so at middle audio frequencies for a mono head and about half this level for a stereo type. The preamplifier must, therefore, provide a considerable amount of voltage gain in order to match this to a hi-fi amplifier, since these require a signal level about 1,000 times higher. It is also necessary for the preamplifier to provide equalization, because the output from a tape head rises at a rate of 6dB per octave. However at higher audio frequencies, tape heads are not very efficient and require a much less rolloff.

Q1 and Q2 are used in a conventional two stage, direct coupled, common emitter amplifier and the frequency-selective negative feed-back through C3 and R4 provides the appropriate equalization. These also set the midband voltage gain of the input stage at about 46dB. With such a low input level it is obviously necessary to use low noise transistors (Such as the BC109C) in order to obtain good results. Running Q1 at a low collector Current, about 200uA, also helps.

Q3 is used as a low gain common emitter stage, which provides the additional amplification. R9 introduces negative feedback, which controls the voltage gain of Q3 and the specified value gives a gain of about 14dB. For a stereo unit R9 should be reduced to 390R in order to give increased gain, to compensate for the lower output of a stereo tape head.

When playing a Dolby B encoded cassette SVV1 can be closed; this gives a small degree of treble cut which provides a reasonably flat overall response.

It is a conventional second order filter circuit having passive high pass filter formed by the series capacitance C3 and C4, plus the parallel resistance of R2 and R3 (the latter also being used to bias emitter follower transistor Q1). A passive filter of this type gives only a very slow initial roll off, and an ultimate attenuation rate of only 6 dB per octave. A bootstrapping resistor is therefore used to improve performance. Above the cut-off frequency, where the gain of the circuit would otherwise fall off somewhat, R1 has the effect of reinforcing the input signal. Well below the cut off frequency, losses through C4 result in the signal level at Q1 emitter being well below that at the junction of C3 and C4. This results in some of the signal at the junction of C3 and being tapped off through R1, with C3 and R1 effectively forming a second high pass filter network. This eliminates the sluts, initial roll off rate (in fact there is a small and insignificant peak of about 0.5dB above the cut off frequency) and speeds up the attenuation rate to a nominal 12dB per octave.

The low pass filter works in much the same way as the high pass one, except of course, the R and C filter elements have been transposed so as to give the correct filter action.

With the specified component values the rumble filter response falls below unity at approximately 45Hz, reaches the -6 d13 point just above 30Hz, and then falls away at a nominal 12dB per octave. The scratch filter response crosses the unity gain point at about 6k5Hz, reaches the -6dB point at approximately 10kHz, and then falls away at a nominal 12dB per octave.

The amplifier is also reasonably sensitive and will give full output with an input of about 50mV. Input impedance is about 50kR.

The slider from the volume control is connected to the, base of Q1 via a DC blocking capacitor. Q1 is connected as a conventional common emitter amplifier with R2 provides the base bias and R3 acting as the collector load. This stage is directly connected to the second transistor which is a PNP type. In this way the current passing through Q1 provides the bias for the second transistor. The output of the second transistor is connected directly to the speech coil of the loudspeaker. This is not normally good practice since the standing current in the output transistor continually biases the coil either slightly in or out from its usual operating point. However if a large speaker is used, as it should be, this has very little effect and, since we are not aiming at hi-fi, it does not matter.

The tone control comprises C2 and RV2 which are connected between the collector and base of Q1 At high resistance settings RV2 has little effect but on minimum settings the 100nF feeds back the high frequencies out of phase, thus cancelling them.

For this circuit to work properly, R3 must be selected with great care. The value shown here of 39 ohms is a typical one and, although it may be used for initial setting up to ensure the circuit is operating, the value should be found by experiment. If it is too low there will be severe distortion at higher volume settings. If it is too high the current drain will be excessive even though the quality of reproduction will be good.

It is very important that Q2 is fitted with a heat sink as it will get very not.

The speaker impedance is not all that critical and in the prototype speakers with an impedance as low as 8 ohms and as high as 80 ohms all worked well, although changing the speaker impedance will also necessitate a change in the value of R3.

The circuit is basically an op-amp used in the non-inverting amplifier mode. The non-inverting input is biased by R4 and R5 via a decoupling network which is comprised of R3 and C3. C4 and C5 give DC blocking at the input and output respectively. With SW1 open there is virtually 100% negative feedback through R1, R2 and C1, giving the circuit unit gain and a flat response. Closing SW1 brings C2 into circuit, and this de-couples some of the feedback through R1 and R2 at frequencies of more than a few hundred Hz, giving the required rising response. Feedback through C1 at high treble frequencies causes the response to fall away above about 5k5Hz, and prevents the very high frequency harmonics from being excessively emphasized.