Document:

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                                                                   EXHIBIT 10.30

December 13, 1999

Patti Hart

President & Chief Executive Officer
Telocity, Inc.
10355 North De Anza Boulevard

San Jose, CA  94014

                              Advertising Agreement

Dear  Ms. Hart:

      This letter sets forth the agreement between NBC Internet, Inc. ("NBCi")
and Telocity, Inc. ("Advertiser") with respect to NBCi's agreement to provide
Advertiser with the right to use certain of NBCi's advertising inventory on NBC
Television Network and its owned and operated television stations (collectively,
"NBC TV") to promote Advertiser only, subject to the following terms and
conditions:

1.    Spots. (a) NBCi shall develop and produce fifteen (15) and thirty (30)
second advertising spots to promote the next generation Internet services
available on the Co-Branded site accessible through Advertiser's high-speed
Internet services (the "Spots"). Advertiser shall reimburse NBCi 25% of all
production expenses for each Spot within thirty (30) days of the completion of
such Spot. Use of each Spot will be subject to Advertiser's approval, not to be
withheld or delayed unreasonably. NBCi will instruct NBC TV to telecast the
Spots on NBC TV on the Dates, Days and Times mutually agreed by NBCi and
Advertiser (subject to NBCi's available inventory and prior sales commitments);
provided, however, that in the event that no such agreement is reached with
regard to the number or value of Spots to be broadcast in any calendar quarter
or year, NBCi may propose and implement a reasonable schedule for the broadcast
of Spots in accordance with the terms of Section 2 below and based upon
Advertiser's reasonable request for such schedule. An initial schedule for the
first quarter of 2000 shall be determined as soon as practicable following the
date hereof. All spots run by Advertiser pursuant to this Letter Agreement shall
be subject to NBC TV's standard terms and conditions for such advertising which
are described in the "Participating Sponsorship Agreement" attached hereto as
Exhibit A (the "Standard Terms") and which are made a part of this Letter
Agreement in their entirety; provided, however, that in the case of a conflict

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between the terms of this Letter Agreement and the terms of the Standard Terms,
the terms of this Letter Agreement shall govern. For purposes of the Standard
Terms, Advertiser shall be both the "Advertiser" and the "Agency" as such terms
are used therein.

         (b) With respect to the placement or telecast of Advertiser's Spots in
any particular Program, Advertiser acknowledges that NBC TV may reject such
placement or telecast if such placement or telecast would compete with or
violate the rights of any other advertiser, sponsor or supplier of such Program
or program category, as determined by NBC TV in its sole discretion and in good
faith; it being understood that NBCi's aggregate commitments set forth in
Section 2 below shall not be affected by any such rejection.

(c) Advertiser may elect to substitute up to 15% of the value of Spots to be
provided by NBCi in any given financial quarter with an equivalent value of
on-line advertising at 100% of the applicable NBCi standard rate card, subject
to the restrictions set forth in Section 7.5 ("Online Promotions") of the
Operating Agreement among NBCi, NBC TV, and Advertiser dated December 13th, 1999
(the "Operating Agreement"), provided that Advertiser gives NBCi three-months
prior written notice.

2.    Value of Spots. NBCi shall telecast Spots with a total spot value of
$13,000,000 (the "Total Spot Value") during the thirty-six (36) months
commencing on January 1, 2000 (the "Effective Date"). The value of each Spot for
purpose of this Letter Agreement shall be calculated at 42.5% of the scatter
market rate in effect at the time such Spot is ordered. The parties agree that
no agency fees or other expenses may be deducted by Advertiser in any way in
connection with determining the number of Shares (as defined below) to be paid
to NBCi pursuant to Section 3 hereof at any time.

3.    Payment for the Spots. (a) Advertiser shall deliver to NBCi 2,480,916
shares of Series C Preferred Stock of Advertiser pursuant to the terms and
conditions of that certain Series C Preferred Stock Purchase Agreement dated as
of the date hereof between Advertiser and certain investors (the "Purchase
Agreement").

      (b)   NBCi shall provide Advertiser with a written report within 10
business days after the end of each calendar month after the Effective Date
during which Advertiser's Spots have been telecast and setting forth the
aggregate value of Advertiser's Spots telecast by NBCi in the preceding month.

4.    Representations and Warranties. NBCi and Advertiser each represent and
warrant that this Letter Agreement has been duly authorized, executed and
delivered by such party and that this Letter Agreement constitutes the legal,
valid and binding obligations of such party, enforceable against it in
accordance with its terms, except as enforceability may be limited by
bankruptcy, insolvency or similar laws affecting creditors' rights generally or
by general

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principles of equity.

5.    Termination. (a) Notwithstanding any other remedy available to NBCi, in
the event that:

            (i)   NBCi notifies Advertiser in writing (with specificity) that
      Advertiser has materially breached this Letter Agreement and Advertiser
      has not cured such alleged breach within thirty (30) days of its receipt
      of such notice; or

            (ii)  upon the occurrence of a Change of Control (as hereinafter
      defined); or

            (iii) Advertiser admits in writing its inability to pay its debts
      generally; makes a general assignment for the benefit of creditors; has
      any proceeding instituted by or against it seeking to adjudicate it as
      bankrupt or insolvent, or seeking liquidation, winding up, reorganization,
      arrangement, adjustment, protection, relief, or composition of Advertiser
      or its debts under any law relating to bankruptcy, insolvency or
      reorganization or relief of debtors, or seeking the entry of an order for
      relief or the appointment of a receiver, trustee, or similar official for
      it or any substantial part of its property; provided, in the case where
      such proceeding is involuntarily instituted against Advertiser, such
      proceeding remains undismissed after thirty (30) days,

then, in any such case, NBCi shall have the right, but not the obligation, to
terminate this Letter Agreement, without prejudice to the rights of the parties
hereunder and, in the event of a termination after NBCi's receipt of the Shares
pursuant to Section 3(a) hereof, NBCi shall pay Advertiser a cash amount equal
to the difference, if positive, between the Total Spot Value and the value of
the Spots already telecast (or deemed telecast), as determined and calculated
pursuant to Sections 1 and 2 above. Notwithstanding the foregoing, the terms
contained in Sections 5, 6, 7 and 8 shall survive the termination hereof. Any
such termination right in connection with a Change of Control shall be
exercisable no later than the later to occur of (x) ten (10) business days prior
to the consummation of such Change of Control and (y) ten (10) business days
after receipt by NBCi of notice (which notice shall identify the third party
having or acquiring Control over Advertiser, be in writing, explicitly state
that it is being delivered in accordance with this Section 5 and provide NBCi
with such additional information as has been provided to the other stockholders
of Advertiser) from Advertiser of such Change of Control (which termination
shall become effective, at NBCi's discretion, upon the consummation of such
Change of Control or following receipt of such notice from Advertiser). For
purposes of this Section 5, the following terms shall have the following
meanings:

            "Change of Control" shall mean (A) any consolidation, reorganization
      or merger of Advertiser with any third party, other than a transaction
      resulting in the holders of the capital stock of Advertiser (prior to such
      consolidation, reorganization or merger) having Control over the surviving
      or resulting entity, (B) any third party having Control over Advertiser or
      (C) any sale, transfer or other disposition by Advertiser of all or

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      substantially all of its assets to any third party; and

            "Control" means the possession, directly or indirectly, of the power
      to direct or cause the direction of the management and policies of
      Advertiser, whether through ownership of voting securities, as trustee or
      executor, by contract or credit arrangement or otherwise.

      (b)   Notwithstanding any other remedy available to Advertiser, in the
event that:

            (i)   Advertiser notifies NBCi in writing (with specificity) that
      NBCi has materially breached this Letter Agreement and NBCi has not cured
      such alleged breach within thirty (30) days of its receipt of such notice;
      or

            (ii)  NBCi admits in writing its inability to pay its debts
      generally; makes a general assignment for the benefit of creditors; has
      any proceeding instituted by or against it seeking to adjudicate it as
      bankrupt or insolvent, or seeking liquidation, winding up, reorganization,
      arrangement, adjustment, protection, relief, or composition of NBCi or its
      debts under any law relating to bankruptcy, insolvency or reorganization
      or relief of debtors, or seeking the entry of an order for relief or the
      appointment of a receiver, trustee, or similar official for it or any
      substantial part of its property; provided, in the case where such
      proceeding is involuntarily instituted against NBCi, such proceeding
      remains undismissed after thirty (30) days,

then, in any such case, Advertiser shall have the right, but not the obligation,
to terminate this Letter Agreement, without prejudice to the rights of the
parties hereunder and, in the event of a termination after NBCi's receipt of the
Shares pursuant to Section 3(a) hereof, require NBCi to pay Advertiser a cash
amount equal to the difference, if positive, between the Total Spot Value and
the value of the Spots already telecast (or deemed telecast), as determined and
calculated pursuant to Sections 1 and 2 above. Notwithstanding the foregoing,
the terms contained in Sections 5, 6, 7 and 8 shall survive the termination
hereof.

(c) In the event that the Operating Agreement is terminated pursuant to Section
12 ("Term; Termination") thereof, NBCi in its sole discretion may (i) terminate
its obligations under this Letter Agreement and provide Advertiser with cash in
lieu of the unused portion of the advertising; or (ii) telecast spots which
promote solely Advertiser and its high-speed Internet services only
("Advertiser-Branded Spot") in lieu of the unused portion of the advertising,
provided that such Advertiser-Branded Spots shall be valued at 100% of the
scatter market rate of a similar spot charged by NBC TV at the time that each
Advertiser-Branded Spot is scheduled by NBCi. In the event that NBCi terminates
the promotional exclusivity set forth in Section 4.12 ("Promotional
Exclusivity") of the Operating Agreement, NBCi will telecast Advertiser-Branded
Spots in lieu of the unused portion of the advertising, provided that such
Advertiser-Branded

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Spots shall be valued at 100% of the scatter market rate of a similar spot
charged by NBC TV at the time that each Advertiser-Branded Spot is scheduled by
NBCi. Advertiser shall bear the total cost of the production of all
Advertiser-Branded Spots. Advertiser agrees that it will comply with NBCi's
then-current policies regarding the timely delivery of commercial materials
related to the Advertiser-Branded Spots.

6.    Miscellaneous. This Letter Agreement, the Purchase Agreement, the
Operating Agreement, and the exhibits and schedules hereto and thereto
constitute the entire agreement and understanding of the parties relating to the
subject matter hereof and supersede all prior and contemporaneous agreements,
negotiations, and understandings between the parties, both oral and written
relating thereto. No waiver or modification of any provision of this Letter
Agreement shall be effective unless in writing and signed by both parties. The
terms of this Letter Agreement shall apply to parties hereto and any of their
successors or assigns; provided, however, that this Letter Agreement may not be
transferred or assigned by Advertiser, including, without limitation, the right
to receive Spots to be telecast by NBC TV, without the prior written consent of
NBCi which shall not be unreasonably withheld. This Letter Agreement may be
executed in counterparts, each of which when executed shall be deemed to be an
original but all of which taken together shall constitute one and the same
agreement.

7.    Governing Law and Jurisdiction. This Letter Agreement shall be governed by
and construed under the laws of the State of New York applicable to contracts
fully performed in New York, without regard to New York conflicts law. The
parties hereto irrevocably consent to and submit to the exclusive jurisdiction
of the federal and state courts located in the County of New York. The parties
hereto irrevocably waive any and all rights to trial by jury in any proceeding
arising out of or relating to this Agreement.

8.    Liability. In the event that NBC TV does not telecast Spots equal to the
Total Spot Value during the thirty-six (36) months after the Effective Date,
then as liquidated damages and not a penalty, NBCi shall pay Advertiser in cash
an amount equal to the difference, if positive, between the Total Spot Value and
the value of the Spots actually telecast, as calculated pursuant to Section 2
above. Except for damages arising out of the gross negligence of willful
misconduct of either party hereto, no party shall be liable to the other party
or its affiliates, officers, directors, successors or assigns for any
incidental, consequential, special or punitive damages or lost profits arising
out of this Letter Agreement, whether liability is asserted in contract or tort
and irrespective of whether it has advised or been advised of the possibility of
any such loss or damage.

      If you are in agreement with the above terms and conditions, please
indicate your acceptance by signing in the space provided below, and return one
original to me. This Letter Agreement shall be null and void if not signed
within two (2) days of the date set forth above.

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                                            Very truly yours,

                                            NBC INTERNET, INC.

                                            By: /s/ AUTHORIZED SIGNATORY
                                               -------------------------
                                                Name:
                                                Title:

ACCEPTED AND AGREED:

TELOCITY, INC.

By: /s/ AUTHORIZED SIGNATORY
   -------------------------
     Name:
     Title:

            [SIGNATURE PAGE TO TELOCITY ADVERTISING LETTER AGREEMENT]<PAGE>   1
                                                                     Exhibit 4.2

COMMON STOCK                                                      COMMON STOCK
                           C-CUBE SEMICONDUCTOR INC.

              INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

                                                             CUSIP

    FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK $.001 PAR VALUE
                                  PER SHARE OF

                           C-CUBE SEMICONDUCTOR INC.

                              CERTIFICATE OF STOCK

/s/ LARRY W. SONSINI                 [SEAL]         /s/ UMESH PADVAL

CORPORATE SECRETARY                        PRESIDENT AND CHIEF EXECUTIVE OFFICER

<PAGE>   2
     A statement of the powers, designations, preferences and relative,
participating, optional or other special rights of each class of stock or notice
thereof and the qualifications, limitations or restrictions of such preferences
and/or rights as established, from time to time, by the Certificate of
Incorporation of the Corporation and by any certificate of determination, the
number of shares constituting each class and series, and the designations
thereof, may be obtained by the holder hereof upon request and without charge at
the principal office of the Corporation.

     The following abbreviations, when used in the inscription on the face of
this certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

     TEN COM -- as tenants in common
     TEN ENT -- as tenants by the entireties
     JT TEN  -- as joint tenants with right of
                survivorship and not as tenants
                in common

     UNIF GIFT MIN ACT -- _____________ Custodian ____________
                             (Cust)                 (Minor)
                          under Uniform Gifts to Minors
                          Act _______________________
                                     (State)

     UNIF TRF MIN ACT -- ______________ Custodian (until age ___)
                            (Cust)
                         ______________ under Uniform Transfers
                            (Minor)
                         to Minors Act __________________________
                                                 (State)

      Additional abbreviations may also be used though not in the above list.

     FOR VALUE RECEIVED, ________________ hereby sell, assign and transfer unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
 IDENTIFYING NUMBER OF ASSIGNEE
-------------------------------------

-------------------------------------

--------------------------------------------------------------------------------
  (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

------------------------------------------------------------------------ Shares
of the common stock represented by the within Certificate, and do hereby
irrevocably constitute and appoint

--------------------------------------------------------------------- Attorney
to transfer the said stock on the books of the within named Corporation with
full power of substitution in the premises.

Dated
     --------------------------------

                    ------------------------------------------------------------
                    NOTICE:  The signature to this assignment must correspond
                             with the name as written upon the face of the
                             Certificate in every particular, without aboration
                             or enlargement or any change whatever.

Signature(s) Guaranteed:

BY:
THE SIGNATURE SHOULD BE GUARANTEED BY AN ELIGIBLE GUARANTOR INSTITUTION (BANKS,
STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND CREDIT UNIONS WITH MEMBERSHIP
IN AN APPROVED SIGNATURE GUARANTEE MEDALLION PROGRAM). PURSUANT TO S.E.C. RULE
17Ad-15.

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