Apparatus and method for calculating signal level in decibels

An apparatus and method for calculating signal level in decibels of a digitally encoded signal. A memory has stored in it a first lookup table with values representing the square of signal amplitude. The memory also has stored in it a second lookup table with values representing signal level in decibels. The signal is sampled for a predetermined number of times and each sample is used to address the first lookup table. The read out square of signal amplitude values are summed and the sum is used to address the second table.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the determination and display of signal level in 
decibels and more particularly to an apparatus and method for performing 
the same. 
2. Description of the Prior Art 
In many applications it is desirable to determine the level of a signal and 
display the same. For example, in testing a digital transmission line 
which carries a digital bit stream in a standard format consisting of 24 
channels, a receiving test unit is connected to the line. The receiving 
unit selects one of the 24 channels and samples the PCM signal in that 
channel to determine among other things its amplitude level in decibels. 
That level is usually displayed along with other information about the 
signal such as its frequency and bit error rate. If a known PCM signal has 
been injected into that channel the information displayed by the receiving 
testing unit will then give an indication of the quality of signal 
transmission on that line. 
As is well known in the art the signal level in decibels can be determined 
by performing the following steps: 
(a) converting the compandored eight (8) bits in the PCM signal to a linear 
signal; 
(b) calculating the square of the amplitude level of that signal; 
(c) adding the squared amplitude level to the squared amplitude level for 
that signal that was calculated in the previous frame in accordance with 
steps a and b; 
(d) repeating steps a, b, and c every 125 microseconds (the time for one 
frame) for a predetermined period of time, e.g. one second, so that a 
large number of samples of signal amplitude level are obtained; 
(e) calculating the level at the end of that predetermined interval of time 
according to the formula 
##EQU1## 
where S.sub.1 to S.sub.N are the samples of signal amplitude level and N 
is the number of samples; 
(f) calculating the level in decibels according to the formula 
##EQU2## 
where 1 mw is the reference level. 
A processing unit such as a digital signal processor can and has been used 
to perform the steps described above. The level in decibels can then be 
displayed in any one of a number of known ways. 
Alternatively and also as is well known in the art the level in decibels 
can be determined by the combination of a codec, RMS converter and A/D 
converter. The calculated level may then be displayed in decibels on a 
meter. The codec, RMS converter and A/D converter each introduce a 
conversion error and therefore the decibel level shown on the meter may 
not be the true level. 
Over the past few years there has been increasing use of hand held battery 
powered equipment in the telecommunications industry to test digital 
transmission lines. Two examples of such hand held equipment are the 
T-TALKER (transmitting) and t-TAKER (receiving) units sold by the assignee 
of the present invention. Both of these units are powered by rechargeable 
batteries and weigh in the order of 19 ounces (540 grams). Such units are 
typically carried by a craftsperson for use in the field. In using such 
units, it may be desirable for the craftsperson to know the level in 
decibels of the signals being received at the receiving unit. Therefore it 
is necessary for that level to be calculated and displayed. It is also 
desirable that such a calculation and display be part of the hand held 
receiving unit without substantially changing the size of that unit. 
As described above the level can be calculated by a digital signal 
processor. Such a processor does, however, use a number of rather 
expensive chip types. It also requires substantially more power for 
operation then is available in the hand held units described above. The 
batteries in the hand held units typically allow for about two to four 
hours of usage before they must be recharged. The level can also be 
calculated by the combination of converters described above. While such a 
combination and display can be packaged in its own portable unit, this 
means that a craftsperson must then use one unit for receiving signals and 
another unit for calculating and displaying levels. Of course, the 
receiving unit and the calculating and displaying unit could be put in a 
single package but the size of that single unit would be substantially 
larger then the size of the hand held receiving unit. 
It is also desireable to not only calculate and display the amplitude level 
in decibels but also the frequency of the signal. The apparatus of the 
present invention is also capable of performing the calculation of signal 
frequency and displaying the same. A hand held receiving unit which 
incorporates the present invention can then calculate and display both 
amplitude level in decibels and signal frequency. 
SUMMARY OF THE INVENTION 
An apparatus for calculating amplitude level in decibels of a digitally 
encoded signal. The apparatus includes a data storage means which has 
stored in a first plurality of locations an associated one of a first 
plurality of values which represent signal amplitude squared. The data 
storage means also has stored in a second plurality of locations the 
associated one of a second plurality of values representing amplitude 
level in decibels. 
The apparatus also includes means for periodically sampling the digitally 
encoded signal a predetermined number of times. That one of the first 
locations associated with each of the samples is addressed and the 
associated first value stored therein is read out. The apparatus further 
includes means which responds to each of the read out first values to 
determine for the predetermined number of times a sum of all of the 
values. Also included are means which respond to that sum for addressing 
the associated one of the second locations so that the second value stored 
therein is read out. 
A method for calculating amplitude level in decibels of a digitally encoded 
signal. The method includes the steps of: 
(a) a periodically sampling the digitally encoded signal a predetermined 
number of times; 
(b) addressing the associated first location to read the amplitude squared 
value stored therein; 
(c) determining for the predetermined number of times a sum of all of the 
read out first values; and 
(d) addressing the second location associated with that sum to read the 
amplitude level in decibels value stored therein.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 there is shown the apparatus 10 of the present 
invention. The apparatus 10 is connected to a digital transmission line 12 
which carries PCM signals in the form of frames. Each frame includes the 
PCM signals for 24 channels and has a duration of 125 microseconds. Each 
channel is made up of eight (8) bits and has a duration of about 5.18 
microseconds. Each frame also includes a single bit known as a 
synchronizing bit which has a duration of about 0.648 microseconds. 
Apparatus 10 includes receiving circuit 14 which breaks the 24 channel 
signal on transmission line 12 into the individual channels. Circuit 14 
determines the occurrence of the start of each frame by recognizing the 
synchronizing bit and provides a pulse indicative thereof. That pulse is 
known as the SYNC signal. Circuit 14 also counts the occurrence of each 
channel in the frame and provides a clock (CLK) signal indicative thereof. 
Circuit 14 may be embodied in the form of an integrated circuit chip such 
as the 8060 type chip available from Rockwell International. 
Apparatus 10 also includes channel recovery circuit 16 which recovers from 
the 24 channels in the frame the one channel whose amplitude level is to 
be determined. Circuit 16 is connected to circuit 14 to receive therefrom 
the 24 channels, the SYNC signal and the CLK signal. Circuit 16 also 
receives the input from a channel selector (not shown), e.g. a thumbwheel 
switch, which is used by the craftsperson to select the one of the 24 
channels in the frame whose amplitude level is to be determined in 
decibels by apparatus 10. 
Circuit 16 includes a counter for counting the CLK signals, i.e. circuit 16 
counts the channels in the frame. Circuit 16 then compares the selected 
channel with the channel count for the frame. When the two are equal, 
circuit 16 allows the eight bits in the selected channel to appear in 
parallel at the associated one of the eight outputs 16a to 16h. It also 
provides at output 16j a STROBE signal which is a pulse occurring once per 
frame at the timeslot associated with the selected channel. Circuit 16 
uses the SYNC signal to reset the channel counter. 
Apparatus 10 further includes processor 18 which is connected to outputs 
16a to 16h and 16j of circuit 16. Processor 18 is programmed to calculate 
the amplitude level in decibels of the selected channel. There is shown in 
FIG. 2 a simplified flow chart of the method by which processor 18 makes 
that calculation. 
In order for apparatus 10 to determine the amplitude level in decibels of 
the selected channel it is first necessary for processor 18 to calculate 
the square of the amplitude represented by the selected eight bits. The 
eight bits may have any one of 256 amplitudes in the range from 0 to 255. 
Therefore the range and values of the square of the amplitudes is known. 
Processor 18 includes a lookup table in which are stored the squares of all 
of the possible amplitudes represented by the selected eight bits. 
Processor 18 uses the selected eight bits and the STROBE signal to address 
an associated one of the locations in that "amplitude squared" lookup 
table. The value in that location is read out and is added to the sum of 
all of the values read out in the previous frames (if any) for that 
channel. Processor 18 is programmed to repeat for a predetermined number 
of frames the process of addressing the amplitude squared lookup table and 
adding the value readout of the addressed location to the sum of all of 
the previously readout values for the selected channel. In one embodiment 
of apparatus 10 the predetermined number of frames was 8000 and processor 
18 was embodied using a type 63701 processor available from Hitachi. 
At the end of the predetermined number of frames, processor 18 has 
calculated the sum of the squares. The signal level in decibels can then 
be calculated from that sum by performing steps e and f of the well known 
prior art method previously described. I have found, however, that it is 
not necessary for apparatus 10 to perform those steps in order to 
determine the level in decibels. 
It is only necessary to include in processor 18 another lookup table in 
which are stored an array of entries, each of which is representative of a 
decibel reading. The number of entries in the table depends on the total 
range of decibel readings to be represented by the table and the desired 
difference in represented decibel readings, i.e. step between each entry. 
The software scans the table unitl it finds that entry in the table which 
is just less than the sum of the squares calculated at the end of the 
predetermined number of frames. The index of the table is related to the 
decibel reading and can be used to calculate the same. That calculation 
involves using one end of the range of second values, the index of that 
location in the table wherein the entry is just less than the calculated 
sum of the squares and the difference in represented decibel reading 
between each entry in the table. It should be appreciated that as the 
decibel lookup table is limited in size there cannot be a unique one to 
one correspondence between a calculated sum of the squares and a location 
in the table. Rather a range of sum of the squares is associated with each 
location. 
In the embodiment of apparatus 10 wherein the predetermined number of 
frames was 8000, the decibel lookup table contained entries representative 
of a total range of decibel readings from -90.0 db to +6.0 db in steps of 
0.1 db. Therefore, the table contained 961 separate sum entries and the 
decibel reading can easily be calculated by adding to -90 db the product 
of the index of that location in the table wherein the stored entry is 
just less than the calculated sum of the squares multiplied by 0.1. It 
should be appreciated that 0.1 is the "decibel" value that each location 
in the table differs from any adjacent location. In other words, apparatus 
10 was able to determine the signal amplitude in decibels with an error of 
no more than 0.1 db. 
Finally, apparatus 10 includes a display 20 which is connected to processor 
18 so that the value readout of the addressed location in the decibel 
lookup table can be displayed. In one such embodiment of apparatus 10, 
display 20 was of the liquid crystal type. 
It should be appreciated that display 20 can also be used to display 
information in addition to amplitude level in decibels. For example, the 
frequency of the signal and bit error rate may also be displayed. 
Apparatus 10 and in particular processor 18 can also be used to calculate 
the frequency of the signal. As described above the eight bits of the 
selected signal appear at outputs 16a to 16h. The eighth bit of that 
signal represents the polarity of the signal. That bit, also known as the 
sign bit, changes its sign twice in each cycle, i.e. for each sample. 
Processor 18 may then be programmed to count the number of sign changes in 
the eighth bit over a predetermined time interval. If that time interval 
is chosen to be one second then in that interval the total number of 
polarity changes is twice the frequency of the signal. The total number of 
changes can then be divided by two to determine the frequency. A 
simplified flow chart for the calculation of frequency is also shown in 
FIG. 2. 
Apparatus 10 may be part of a receiving unit (not shown in FIG. 1). That 
receiving unit may be of the hand held type previously described. The 
channel selector is typically mounted on the unit. Apparatus 10 embodied 
using the liquid crystal display is ideally suited for incorporation in 
such hand held type receiving units as even with the display, apparatus 10 
does not substantially change the size of the hand held unit. It also does 
not substantially change the load presented to the rechargeable batteries 
in such units. 
It is to be understood that the description of the preferred embodiment is 
intended to be only illustrative, rather than exhaustive, of the present 
invention. Those of ordinary skill will be able to make certain additions, 
deletions, and/or modifications to the embodiment of the disclosed subject 
matter without departing from the spirit of the invention or its scope, as 
defined by the appended claims.