Definition of level and attenuation in telephone networks

Transcription

1 Defnton of eve and attenuaton n teephone networks o The purpose: defnton of the measurement unts used for sgna eve and crcut gan/attenuaton n teephony; defnton of the reference ponts empoyed n teephone networks; o Dgta exchanges are characterzed by compex mpedance at the nterface wth the subscrber ne the defntons are generazed for powers expressed n mva nstead of mw; 1. Attenuaton/oss and gan (A V T E L E H O N E C IR C U IT V o o T es ted crc u t Fg. 1 Defnton of attenuaton 1 o Rato of powers (rea or apparent: A 1 g [ db] = (1 ; f A s postve, t s about power attenuaton/oss ( < 1 ; f A s negatve, t s about a power gan ( > 1. V1 A = g db (; f A s postve, t s about votage attenuaton/oss (V <V 1 ; f V o Votage rato: A s negatve, t s about a votage gan (V >V 1.. Return oss (A r o The return oss expresses the degree of msmatch between two mpedances 1 and and t s gven 1 + by the reaton: Ar = g 1 (3; ths formua represents the attenuaton expressed n db between an ncdent and the refected sgna, n an mpedance msmatchng pont. 3. Apparent power attenuaton/oss (A a E ~ V TELEHONE CIRCUIT V o o E ~ x Tested crcut Fg. Defnton of apparent power attenuaton Reference crcut o The nput apparent power s defned as the power generated on a oad mpedance equa wth the E nterna mpedance of the generator; = (4; 4 f power o s the output apparent power on oad mpedance, that s: apparent power attenuaton s gven by: Vo o = (5 ( f E Aa f = 1 g = g db o Vo f (6

2 The genera formua s appcabe n the case of passve crcuts fters and passve recproca crcuts; n ths case the attenuaton concept s based on the rato between the nput and output apparent power (dentca defnton n both drectons recproca character; If the mpedances are resstve reaton (6 can be apped; f we have a chan of ampfers (for ex. ampfers desgned wth operatona ampfers see fg. 3 whch are not nfuenced by E the mpedances and, condton g = ct s fufed and reaton (6 must be Vo modfed (for mantanng the genera concept of apparent power transfer: The apparent power attenuaton s computed at the reference frequency f =1Hz: ( f E Aa ( f = g db V f (7 The apparent power attenuaton s expressed accordng to frequency as n (6. Because the rato of mpedances s ndependent of frequency, attenuaton varaton wth the frequency depends ony on the E/V o rato R V + V R 1 AO V V Inserton attenuaton/oss (A Fg. 3 Crcut composed of a chan of ampfers o The nput apparent power s defned as the power generated on an mpedance equa wth the oad mpedance: E + = f f f power o s the output apparent power on the oad mpedance, that s: (8; the nserton attenuaton s gven by: E A = 1 g = g db o Vo + ( f (1 Vo o = (9 As n the case of the apparent power attenuaton the genera formua s appcabe n the case of passve crcuts fters and passve recproca crcuts; f we have a chan of ampfers (for ex. ampfers desgned wth operatona ampfers see fg. 3 whch are not nfuenced by the mpedances and, condton g E = ct s fufed and reaton (1 must be V modfed (for mantanng the genera concept of apparent power transfer: Inserton attenuaton/oss s computed at the reference frequency f =1Hz: ( ( f + E f A ( f = 1 g = g db o Vo f f f (11

3 Inserton attenuaton/oss s expressed accordng (1. Because the rato of mpedances s ndependent of frequency, attenuaton varaton wth the frequency depends ony on the E/V rato. 5. Absoute power eve expressed n dbm for test tones o Represents the apparent power eve of a sne sgna expressed n db, reatvey to a 1mVA apparent V n ( f power: L = 1 g = 1 g [ dbm] 1mVA 1mVA (1 o In anaog networks s usua the 1mW reference power; on the anaog nterfaces of the dgta exchanges (two wre nterfaces compex mpedances are used, as a rue t s necessary to use the apparent power; mpedances are evauated at the reference 1Hz frequency. 6. Absoute power eve of the rea sgnas o It s about the voce sgnas havng a 3Hz-34Hz frequency band; there are the foowng cases: f V Systems wth resstve mpedances: the actve power s computed as: = df [ mw ](13 R where R s the oad resstance n Ω, V (f/r s the power spectra densty n mw/hz, f 1, f are the mt of the frequency band n Hz; the absoute power eve n ths case s: L = 1 g [ dbm] (14 1mW Systems wth compex mpedances: the actve power s computed as: f V = df [ mva] f (15; the absoute power eve ths case s: L = 1 g [ dbm ] (16; 1mVA f1 the varaton of the power eve wth frequency s dependent ony on the votage. 7. sophometrc power eve (dbmp o hysoogca characterstcs of the ear make the percepton of the sound dependent on the frequency accordng to a weghtng functon W(f, the psophometrc characterstc; the effect of the ow frequency (for ex. 5Hz or hgh frequency (3Hz components s ower than the effect due to the components ncuded n the 8Hz Hz band; f we take nto account ths weghtng characterstc the expresson of the absoute and reatve power becomes: f V p = f1 8. Reatve power eve (dbr = (18 1mVA W 1 p 1 df [ mva] (17 L 1 g [ dbmp] p ( 1Hz [ db] Hz o The reatve eve (n db n a pont s gven by: L r = 1 g (19; and are the 1 apparent power n the consdered pont respectvey n the reference pont TR (Transmsson Reference ont; n the case of some equpments, ke BX, t s usua aso the term of LR (Leve Reference ont. The reatve eve s numercay equa wth the attenuaton/oss or gan between the consdered and the reference pont at 1Hz frequency; n the reference pont the reatve eve s dbr; The measurement sgna apped n the transmsson reference pont (dbr has, as a rue, a vaue of -1dBm, to avod the rsk of systems overoadng; Sgnfcance of the reatve eve: t aows the characterzaton of some propertes ke: gan or attenuaton between nterfaces and of the capabty of certan equpment to process the sgnas apped to an nterface; Can be used aso n equpment desgnng and n performance tests. f1

4 9. Leve expressed n dbm o Represents the absoute power eve measured at 1Hz reference frequency n the transmsson reference pont (dbr; n a gven pont havng the reatve eve L r, a eve L dbm generates an L = L L dbm ( absoute vaue L a : a + r -5dBr -14dBr dbr dbr 4dBr 14dBr 1. The use of the reatve eve (dbr and of the dbm eve n equpment desgnng and n performance tests the case of CM systems o For the evauaton of the coders and decoders of the CM systems the concept of dgta reference sequence was ntroduced (Dgta Reference Sequence DRS; DRS s a possbe CM code sequence whch, by decodng wth an dea decoder, generates a dbm sgna. Inversey, an anaog sgna wth a dbm eve apped at the nput of an dea coder w generate a reference dgta sequence; the frequency of the sgna (a sne sgna s chosen to be 1kHz (DRS represented by a sequence of 8 sampes, but coud be aso seected a frequency whch s not a sub-harmonc of the 8kHz sampng frequency an approprate vaue s 1Hz, but n ths case the DRS sgna generaton w be more compex; DRS obtaned from the reference sne sgna has a peak vaue of 118; the maxmum codng eve n the case of an A aw coder s 3,14dBm for a sne sgna. o The process of reatve eves computaton for the case of rea coders and decoders, when the dgta path doesn t ncude any dgta processng (for ex. dgta attenuaton t s depcted n fg. 5. By defnton the reatve eve on such dgta nk s dbr. The computaton of reatve eves n ponts s and r t s accompshed n the foowng ways: t s modfed the eve n pont s unt a devce connected to the dgta path dentfes the DRS sequence correspondng to a eve of dbm; f the measured eve s S dbm, resuts a reatve eve of S dbr=s dbm dbm; smary, n the case of the decoder, a DRS sequence s apped to the decoder and an R dbm eve, numercay equa wth the reatve eve expressed n dbr, s measured at the output of the decoder. 11. Other parameters expressed n unts derved from db the votage eve U u = (1, 775mV where U x s the effectve vaue of the measured votage; 775mV s the votage whch produces a power of 1mW on a 6Ω resstor; the votage eve s n genera dfferent of the power eve; the dfference appears f the oad mpedance s compex and the moduus s not equa wth 6Ω, and we x o By defnton, the votage eve expressed n dbu s gven by the reaton: L g [ dbu] 6 = u ( have: L L +1 g [ dbm] p 9dB 14dB 4dB 18dB -15dBm -4dBm -1dBm Fg. 4 Defnton of absoute and reatve eves ~ s dbm A/D S dbm DRS Detector -1dBm DRS Generator -6dBm r R dbm -4dBm Fg. 5 Use of the DRS sequence for computaton of the reatve eves ocated n front of the CM coder and behnd the CM decoders D/A dbm

5 1. Vrtua Internatona Connectng ont VIC o The VIC ponts defne the mts between the natona and nternatona part of a connecton (see fg. 6; are used as reference ponts for the natona and the nternatona parts; VIC VIC c a H 1 b c ta c t1 c t c tb b H c b Natona network a Internatona network Fg. 6 Defnton of the vrtua nternatona connecton ponts Natona network b 13. Loudness ratng (LR o A teephone communcaton suppose the exstence n the end ponts of two basc eements: one of them transmts the sound message (the mouth, the other receves the sound message (the ear ; a goba teephone communcaton supposes an acoustc pressure at the both ends of the connecton and an eectrc sgna used to transmt the sound message aong the connecton; the correct pannng of the teecommuncaton networks requests the expresson of the eectroacoustc attenuaton between the acoustc source and the recever; the specfc attenuaton (n db, obtaned by estabshng equvaence between the attenuaton of the two type of sgnas s the so caed oudness ratng (LR; f the crcut s dvded n severa bocks, then the goba ratng for oudness s the sum of the ndvdua vaues of LR. o It s consdered as reference for the natona system the VIC pont and the LR parameter s defned by the components: Loudness ratng at transmsson (LRT represents the attenuaton between the mouth of the subscrber who s speakng and the VIC pont (or other reference pont n ths case the oudness ratng represents the weghted mean (n db of the acoustc exctaton pressure reported to the measured votage (or power; Loudness ratng at recepton (LRR represents the attenuaton between the VIC pont (or other reference pont and the ear of the subscrber who s stenng n ths case the oudness ratng s defned as the weghted mean (n db of the exctaton votage reported to the measured acoustc pressure (see fg. 7. Remark: the oudness ratngs LRT and LRR can be computed at any eectrca nterface of a teephone network. o In fg. 7 t s presented the compete schematc of a teephone connecton between the sound source and the recever, composed of severa nterconnected parts; we have the foowng reatons: n n (3 = 1 = 1 EFE = EFE phone + EFC ; EFR = EFR phone + EFC ; EFG = EFE + EFR EFE-phone EFC 1 EFC k EFC n EFC n+1 EFC s EFC m EFR-phone EFE EFR EFG Fg. 7 Loudness ratngs defned for a teephone connecton Reference pont

6 o The computaton of the oudness ratngs can be reazed n subjectve manner ow performances, or n objectve manner t s necessary a psycho-acoustc mode whch smuates the modaty n whch the bran transate the sonorous mpressons; o The oudness ratngs of the teephone devces are obtaned by usng speca equpments t s not necessary a very precse measurement of the rea oudness ratng because ths depends on the subscrber; the oudness ratng of a crcut s equa wth the attenuaton at 1Hz f the attenuaton s constant n the whoe bandwdth, respectvey the mean attenuaton n the 3Hz-34Hz band f there are mportant attenuaton dstortons; the oudness ratng for the subscrber crcuts can be computed usng the foowng reaton: EFC = K L R C (4, where R s the cabe resstance n Ω/km, C s the cabe capacty n nf/km, L s the ength n km, K s a constant that depends on the termna mpedance of the cabe (K=.14 for =6Ω, K=.15 for =9Ω, K=.16 for -compex. o In norma condtons of expotaton a goba vaue of 1dB for the oudness ratng s acceptabe. 14. Data sgna eve on mxed crcuts o The power eve of the sgnas transmtted and receved by modems has a fundamenta mportance n the case of data transmssons performed on teephone crcuts; the seecton of a reasonabe vaue of the data sgna eve s nfuenced by two opposte condtons: vaues as hgh as possbe necessary to ensure sgna to nose rato as hgh as possbe at recepton; moderate vaues, approprate for avodng cross-taks rsks and sgnas whose peaks can overrun the nearty threshod of the CM converters; o It s mposed a maxmum power eve of dbm and a mean power eve of -13dBm; for eves receved by modems ntended for good quaty eased nes t s mposed -6dBm; -8dBm for modems ntended for medum quaty eased nes; -43dBm for modems ntended for swtched nes; 15. Evauaton of the nose parameters There are used dfferent weghtng fters (characterstcs for dfferent stuatons n order to evauate the performances of teephone transmssons. o The non-weghted frequency characterstc (fat characterstc t s used for the evauaton of dfferent equpment performances and for the evauaton of the data transmssons reazed on teephone crcuts. o The non-weghted characterstc s a ow pass characterstc wth a 3kHz bandwdth; the characterstc s fat up the 5Hz frequency to be possbe the evauaton of the effects of the power grd frequency on teecommuncaton nes. o The narrow band frequency s used for the dentfcaton and evauaton of nterferng sgnas wth dscrete frequences (dfferent tones. o The psophometrc characterstc (fg. 8 t s a speca weghtng (fterng characterstc used to the evauaton of the nose on the transmsson of voce sgnas t s consdered the frequency characterstc of the ear. Spectra components of the nose ocated n the 8Hz-5Hz band are the most dsturbng, the senstvty of the ear beng the hghest n ths frequency band. The senstvty of the ear decreases at frequences ocated beow 3Hz and above 4Hz, the nose components ocated beow and above these threshods havng a reatvey reduced effect. The absoute weghted power eve (psophometrc eve s expressed n dbmp.

7 -5 Reatve weght (db (+ mt (- mt -45 frequency (Hz og. scae Fg. 8 Spectra mask of the psophometrc weghtng fter